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DOI 10.1007/s12182-011-0147-8
Controls on reservoir quality in the paleogene Kalatar Formation of the southwestern region of the Tarim Basin, China Yang Haijun2, Shen Jian-Wei1 , Wang Xu1, 3, Zhang Lijuan2, Li Meng2 Key Laboratory of Marginal Sea Geology of CAS, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China 2 Exploration and Development Research Institute, Tarim Oilfield Company, Petroleum Company Limited, Korla 841000, China 3 Graduate School of the Chinese Academy of Sciences, Beijing 100049, China 1
© China University of Petroleum (Beijing) and Springer-Verlag Berlin Heidelberg 2011
Abstract: The Paleogene Kalatar Formation is a main target for petroleum ex125 ploration in the southwestern region of the Tarim Basin (SWRTB) and a systematical summary of controls on reservoirs of high quality has important implications for this area. According to outcrop analysis, as well as core, and laboratory experiments based on theories of sedimentology, it can be inferred that the main pore types in the Kalatar Formation are moldic, vuggy, intercrystalline and interparticle pores, fractures, and mudstone microporosity, respectively. The foreshore in the shore-shelf depositional model is the most favorable target, wherein grainstone and dolomitized grainstone are characterized by high quality in a middle ramp subfacies of the carbonate ramp model. Diagenetic factors, such as micritization, cementation, compaction, neomorphism, silicification, and pyritization, are detrimental for reservoirs. At the same time, leaching and dolomitization can improve the quality of reservoirs. Permeability of reservoirs can be greatly improved by fractures, through which quality of reservoirs can be effectively enhanced. Concentration of CO2, temperature, and humidity are affected by changes in climate, which can have an influence on depositional setting, composition and diagenesis of sediments, and eventually the properties of reservoirs. The hot and arid climate of the Paleogene was harmful to development of reservoirs, which is demonstrated in the contemporaneous diagenetic and epigenetic stages. Shallow and deep-burial diagenesis are closely related to the formation and the connate water, however these are rarely affected by paleo-climatic variation. There is possible erosion that mainly resulted from organic acid in relative well intervals according to indications of hydrocarbons in the early diagenetic stage. Fracture systems can be established when acidic fluids are emplaced during periods of uplift, by which pores can be effectively enlarged during the late diagenetic stages.
Key words: Kalatar Formation, sedimentary facies, diagenesis, dolomitization, fractures, climatic conditions, Tarim Basin
1 Introduction Development of reservoirs is controlled by many kinds of factors, such as sedimentary facies, diagenesis, sequence and sea level variation, tectonic activity, basal paleo-uplift, paleokarstification, and paleoclimate (Lucia, 1995; Ma et al, 1999; Saller et al, 2006; Harris, 2010). The time of deposition of the Paleogene Kalatar Formation corresponds to that of the Himalayan orogenic movements in the southwestern region of the Tarim Basin (SWRTB). Marine distribution, *Corresponding author. email:
[email protected] Received December 17, 2010
sediment supply, differentiation of depositional environments, distribution of sedimentary facies, relative sea-level change, sequence, and diagenesis in the SWRTB basin were all greatly influenced by the development of fold belts in the Kunlun and South Tian mountains, which represent the main controls on the quality of reservoirs from tectonic activity. Other controls of a regional scale also play a role under these fore-mentioned categories (Wang et al, 2002; Sun et al, 2005; Zhou et al, 2005). Controls on the quality of the Kalatar Formation recorded in past include sedimentary facies, diagenesis, tectonic activity, and dolomitization (Wang, 2000; Shao et al, 2006; Shao et al, 2007). But these studies are regionally restricted within the Kashi sag or Well
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2 Geological setting
Keshen 103 and in the Keliyang area. These influences are short of a systematic and comprehensive summary of controls on the Kalatar Formation in the whole SWRTB. Influences on reservoirs from structural fractures and climatic change are not previously regarded, either. The Paleogene Kalatar Formation is the main hydrocarbon exploration target in the SWRTB (Jia, 1997). Controls on reservoirs of high quality can be acquired through systematical analysis of the outcrop, logging, and laboratory experiments on a whole regional scale, which has important theoretical and practical ramifications for prediction of hydrocarbon distribution and exploration as well as production of reservoirs with higher efficiency. Controls on the quality of the Paleogene Kalatar Formation can be systematically and objectively summarized in terms of perspectives such as sedimentary facies, diagenesis, structural fractures, dolomitization, climatic conditions, and forward modeling, which can be effectively applied to help guide exploration in the SWRTB.
Sitting on the pre-Paleozoic basement with a north-west strike, the SWRTB depression has an important tectonic component, which reflects superposition of a Paleozoic cratonic basin with a Mesozoic-Cenozoic foreland basin (Jia et al, 1997). Depth of basement in the SWRTB depression ranges from 9 to 16km, and is affected by a central uplift, called the Tiekelike uplift. Also bounding the basin are the Keping uplift ,Tianshan foldbelt, and Mingfeng faulted uplift in northeast, southwest, north, and southeast, respectively. There is a series of faults cutting through basement rocks with a northwest-west strike in the depression, which can be subdivided into the Kashi, Yecheng, and Hetian depocenters from west to east. These features reflect basement depths ranging from 12km to 16km. The SWRTB depression is composed of four subtectonic units, which are the Kashi sag, Yecheng sag, Hetian sag, and Maigaiti slope,
South Tian Mountain Fold Belt
Keping Uplift
Wulukeqieti
Ka ngsu
Sim uhana Kukebai
Kuzigongsu
Tashipisake Atus hi
Wuqia
Maigaiti
4400000
Wulageng Bachu
Kab ieled ab an Tuom uluoan
Kas hi
st
Wuyitagegaizi R iver
we
Aketao
N
Uplift
uth
Sufu
Kunlun
Sule
So
Wubulate
J ias hi
Yuepuhu
Kashi Sag
Slope
Ta r
Yingjis ha Kus han R iver
im
M aigaiti 4300000
0
50
Yigeziya
Mountain
100km
De
Tongyouluke
pr
Tam u River Qim eigan
es
si o
n
Shache
Fold
Yecheng Sag Zepu
Ganjiate Aertas hi
Urumqi
Yecheng
Xingjiang Uygur Autonomous Region
Hes hilafu
4200000
Belt Hetian
Qipan
Sag
ike
kel
Ti e
Kashi 400Km
Saigertas hi Kekeya Yukeyang
Up Surveyed section 13400000
Detailed measured section 13500000
Tectonic boundary line
City or town 13600000
Keliyang
lift 4100000 13700000
Fig. 1 Schematic map showing tectonic units, surveyed/detailed measured sections and well locations in the SWRTB depression. The enlarged area is indicated within the red-dashed square in the lower left corner (modified after Jia et al, 1997)
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respectively (Fig. 1). The Paleogene Kalatar Formation is mainly distributed in front of the South Tian and Kunlun mountains in the SWRTB depression. Collision between the Indian and Asian plates along the Yaluzangbu River is the main factor that controlled Paleogene sedimentation in the SWRTB depression (Ding et al, 1997). Sedimentation can be subdivided into two deformation stages: strike-slip in the early stage and compression in the late stage with the Oligocene as a transitional period. Present tectonic style on the periphery of the SWRTB orogene was established during the Oligocene when collision between Indian and Asian plates changed from strike-slip mechanics along plate margins to intensively compressional thrusting and thickening with an accompanying uplift of the interior Qinghai-Tibet Plateau (Wang, 2004).
3 Lithology of the Kalatar Formation The SWRTB is adjacent to the Kunlun and South Tian mountain fronts. Due to influx of terrigenous sediments from the Kunlun Mountain, effects from the front belt of the South Tian Mountain, a hot-arid climate in the subtropical zone (Scheibner and Speijer, 2008), and tectonic activity, a large variety of rock types with complex origins in the Paleogene Kalatar Formation are deposited in study area. These are composed of terrigenous clastics, carbonates, and mixed siliciclastic and carbonate rocks. In addition, evaporites formed under an arid environment are also a common rock type in the Kalatar Formation. On the whole, rock types change from terrigenous clastics to carbonates from mountain fronts to the basin (Shen et al, 2010). Dolomite in the Kalatar Formation mainly occurs in four areas, which are the Kekeya area (Heshilafu, Aertashi, and Ganjiate sections), periphery of the Qimugeng uplift (Tongyouluke and Qimeigan sections), the Maigaiti Slope (Well Tacan 2, Qun 6, and Qun 601), and northern part of the Kashi sag (Kangsu and Kukebai sections), respectively (Fig. 1, Table 1). The Kalatar Formation has different lithological assemblages in different
areas (Table 1). Great differences exist in how formational standards are discriminated in different areas. There is an obvious lithological difference between the Kalatar Formation and the overlying the Qimugeng Formation. Because of depositional continuity leading to lithological similarity between the Kalatar Formation and the underlying Wulageng Formation, it is more difficult to distinguish their boundary than that between the Kalatar Formation and the overlying Qimugeng Formation (Sun, 2001; Zhang et al, 2002; Wang, 2004). Carbonate rocks in the Kalatar Formation are mainly dominated by medium-high energy shoal calcsparite oolitic dolostone, calcsparite psammitic oolitic dolostone, psammitic dolarenite, microsparry skeletal dolostone, calcsparite oosparite, calcsparite oolitic arenitic limestone, calcsparite bioclastic oolitic limestone, and calcsparite bioclastic limestone in a shallow ramp depositional environment. Calcsparite oolitic dolostone is texturally composed of oolite with quantities ranging from 50% to 80%. This oolitic dolostone is characterized by elliptical or circular morphologies with granule diameters ranging from 0.4mm to 0.8mm, concentric laminations or pellicular oolite, and little radial concentric oolites with bioclastics such as foraminifer and terrigenous clastics as their oolitic cores. Amounts of sand and bioclastics, calcsparite cements, and micritic calcite and anhydrite found as interparticles are less than 1% to 25%, ranging from 20% to 25%, and commonly no more than 5% respectively (Shen, 2010). Clastics in the Kalatar Formation are usually mingled with carbonate rocks in the transitional mixing facies, which are commonly mixed and interbedded with bioclastics, oolites, and sands. The quantity of intraclasts amounts to from 5% to 25% of the total amount of clastics, which shows intraclastics such as fine feldspathic lithic sandstone and fine lithic sandstone gradually transform to sandy calcsparite bioclastic limestone and calcsparite psammitic limestone. Limy fine feldspathic lithic sandstone is mineralogically composed of 55% to 65% quartz, from 10% to 12% feldspar, and from 25% to 33% rock fragments.
Table 1 Lithology of the Paleogene Kalatar Formation, the overlying Wulageng Formation, and the underlying Qimugeng Formation in the SWRTB Stratum Series
Formation
Uplift of Kekeya and Qimugeng
Southern Part of Kashi Sag
Northern Part of Kashi Sag
gray green calcareous mudstone and gray sandstone
gray green silty mudstone brown calcareous siltstone
gray green mudstone
Shell limestone
Sandstone and mudstone
Muddy sandstone mingled with shell limestone
Middle
Gypsum sandy mudstone
Shell limestone, muddy sandstone, pellet and oolite bearing micritic limestone
Shell limestone grainstone
Lower
Grainstone, granular dolomite
Sandstone and mudstone
Sandstone, mudstone, dolomite and oolitic dolomite
mauve calcareous mudstone
mauve calcareous mudstone
mauve calcareous siltstone and calcareous mudstone
Member
Wulageng
Upper Eocene Kalatar
Palaeocene
Qimugeng
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Feldspar and rock fragments are lithologically dominated by potassic feldspar and metamorphic rocks, respectively (Wang, 2004). Grains are characterized by a subangular shape, dotline contacting boundaries, and interstitial cementation.
4 Samples and methodology Field samples were mainly collected from four surveyed sections and eleven detailed measured sections in the SWRTB (Fig. 1). Observation and description of cores were completed in the Core Repository of the Tarim Oilfield Company. The method proposed by Lucia (1999) was adapted to shift correction of core migration. Common and paleontological thin sections were polished in the Second Geological Crew of Bureau of Mineral Exploration and Development facility of the Xinjiang Autonomous Region, and stained with alizarin-red S in order to differentiate dolomite and calcite (Dickson, 1965; Azmy et al, 2009). Additionally, some of these thin sections were stained with potassium ferricyanide. Polishing and examination of moldic thin sections and analysis of porosity and permeability of plugs were completed in the Rock and Mineral Laboratory of the Tarim Oilfield Company. Some typical thin sections selected were examined by scanning electron microscopy (SEM) in the SEM laboratory of the South China Sea Institute of Oceanology, Chinese Academy of Sciences (HITACHI S3400, at 20 KV, and 10 mm working distance). Cathodoluminescence (CL) experiments were performed in the Petroleum Exploration and Development Laboratory of Hubei province, China University of Geoscience (Wuhan) (CL model, voltage used, current, pressure and microscope model are English brand cl8200mkd, 10 KV, 0.5 mA, 10 Pa, and Leica dm2500p, respectively). Logging of data including volume of shale, deep and shallow resistivities, and gamma data were accomplished at the Logging Centre of the Institute of Petroleum Exploration and Production, Tarim Oilfield Company, all of which have been subject to environmental correction, basal line shifting, and normalization of matrix (Westphal et al, 2004; Aigner et al, 2007).
5 Types of pore Pores in carbonate rocks can be divided into two types; primary pores formed during the process of sedimentation and secondary pores developed through diagenesis, the latter of which is the main goal of study (Read, 1985; Frost et al, 2009). Based on observations of thin and cast sections acquired from four surveyed sections, seven observed sections, and nine wells (Fig. 1) in combination with scanning electron microscopy (SEM) and cathodoluminescence (CL), it is concluded that type of reservoir accommodation in the Paleogene Kalatar Formation is dominated by secondary pore development. Small fractures and vugs are the main types of porosity, which make reservoirs characterized by pore-fissure migration pathways. There are many kinds of pores in the Kalatar Formation, such as moldic, erosional, intercrystalline and, interparticle pores, fractures, and mudstone microporosity (Arve, 2006) (Fig. 2).
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6 Controls on the quality of reservoirs 6.1 Sedimentary facies Reservoirs are directly controlled by sedimentary facies, which control rock type and strata thickness that reflect important characteristics of reservoirs (Ma et al, 1999; Harris and Vlaswinkel, 2008). Properties of reservoirs in the Kalatar Formation are obviously influenced by facies on the basis of measurements and statistics of properties from different facies in SWRTB. Two kinds of sedimentary model can be summarized referring to characteristics and distribution of sedimentary facies in the Kalatar Formation, which are shore-shelf sedimentary model and carbonate ramp model, respectively. 6.1.1 Shore-shelf sedimentary model This model is composed of backshore, foreshore, offshore, and shelf environments (Fig. 3). The foreshore is characterized by lithologies that include conglomeratic quartz sandstone (Aase and Walderhaug, 2005), lithic quartz sandstone, siltstone, and fine lithic sandstone. Subsequent is the offshore subfacies characterized by abundant rock types such as bioclastic muddy sandstone, calcareous mudstone, calcareous fine-silt lithic sandstone, and sandy micritic bioclastic limestone. The most favorable target in the shoreshelf model is the shelf setting , the petrology of which includes mudstone, siltstone, muddy shelly limestone, and micrite (Fig. 4). 6.1.2 Carbonate ramp model The carbonate ramp model adapted for this study consists of three sedimentary subfacies, which are the inner ramp, middle ramp, and outer ramp, respectively (Read, 1985; Claps et al, 2009) (Fig. 5). The whole interval of the Kalatar Formation in the Keliyang section and lower interval of the Qimeigang section is represented by inner ramp deposition. Lithologies include siltstone, calcareous mudstone, and gypsum. Sedimentation on the middle ramp is characterized by grainstone, granular dolomite and dolomitized grainstone with a little micritic grainstone and grain micrite in the Qimeigang, Tongyouluke, Kukebai, Wuheshalu, and Kangsu outcrop sections. The outer ramp is dominated by wackstone and micrite. Properties of reservoirs in the middle ramp are better than those developed in the outer ramp (Fig. 6). Cements of different types in the middle ramp have much influence over the quality of reservoirs (Fig. 7). The sequence of reservoirs with different cements from best to worst are dolomitized oolitic limestone (Conliffe et al, 2009), oolitic or bioclastic limestone cemented by calcsparite, micriticcalcsparitic limestone, and micritic limestone, respectively. The volume of shale in the shallow shoal microfacies is lowest, the next lowest is deep ramp, the third is tidal flat, and the highest is lagoon facies. Deep resistivity in four microfacies gradually decreases by turn, as above. A variable range in values is related to the different quantities of samples collected. Box and whiskers plots can be established and be used for interpretation and determination of different subfacies by electronic data (Stacy et al, 2010) (Fig. 8).
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Average high tide level
e
un
Average low tide level
Tid a
e
ur
n du
Artashi section
Ti tro dal ug sc h o
tal
as
Co
Heshilafu section
Wave base
stal Coa bar d n a s
el
l
via
u All
fan
nn
as
Co
lc ha
d tal
Ganjiate section
Wubulate Kalabieledaban section section Tidal channel
Average high tide level
Rock types
is
br
De
Average low tide level
w
flo
Wave base
Conglomeratic Quartz sandstone Rock quartz Sandstone Mudstone Siltstone Fine sandstone
Backshore
Sedimentary subfacies
Foothill belt Alluvial fan
Fine granular Rock sandstone Bioclastic mudy Sandstone Argillaceous shelly Limestone Micrite Foreshore
Shoreface
Shelf
Fig. 3 Shore-shelf sedimentary model of the Paleogene Kalatar Formation in the SWRTB
Porosity
5
Plane porosity
4 3 2 1 0
Foreshore
Shoreface shore subfacies
Shelf
a
Average permeability, md
Average porosity/plane porosity, %
6
3 2 1 0
Foreshore
Shoreface shore subfacies
Shelf
b
Fig. 4 Average properties analysis of rocks within different shore subfacies of the Paleogene Kalatar Formation in the SWRTB. a) Average porosity and plane porosity histogram; b) Average permeability histogram
and pyrite in the late burial stage, most of the cements in carbonate sediments are also carbonate (Lucia, 1999). The Kalatar Formation is characterized by most of the limestone interval deposited under low energy conditions with relatively rare calcsparite. Interstitials of grains are commonly cemented by micrite after deposition, the resulting porosity of which is greatly reduced, and a detriment to the quality of reservoirs. During the period of grainstone sedimentation, interparticle pores and cements in the interstitials are all well developed under high-energy conditions. There are two stages of calcite cementation with high impact on reservoirs, as based on analysis of characteristics and settings of cements, which are submarine cements in the submarine diagenetic setting characterized by micritic fibrous calcite and coatings, as well
as fresh water cementation characterized by even granular and coarse calcite crystals, respectively (Fig. 9B; Fig. 10) 6.2.3 Leaching Leaching is an important factor in reservoirs of high quality, which reflects constructive diagenesis in improvement of porosity and permeability (Moore, 2001). There occur both contemporaneous fresh water and burial leaching in the Kalatar Formation, according to observations by microscope. The contemporaneous or pene-contemporaneous leaching is occurring in a fresh-water digenetic setting. Grainstone from a supratidal and shallow shoal depositional setting is subjected to leaching in the lowstand system tract controlled when sea level is falling (Fig. 9C; Fig. 10). There is an obvious relationship between freshwater leaching and
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s)
Kangsu section
Wuheshalu section
Kukebai section
Tongyouluke section
Keliyang section
Qimeigan section
308
Inner ramp
Middle ramp
Lagoon
Tidal flat
) Slo Ca pe tac lith las olo tic gy roc (lim k, e an ston d s e, lum sh p s ale, tru ctu re
ha
lc
a Tid
Mean sea level
B ec ioc an hin last d at ics gr e, an an (co ul d qu ar g in sh ast oid oa ro , l po do u
l
e nn
Outer ramp
Fair weather wave base Storm wave base Storm energy Rock type
Dolomite
Rock texture
Siltstone,(anhydritic, sandy,calcitic) mudstone, gypsum
Matrix support
Limestone
Mudstone Grain support
Muddy limestone/Micrite
Wackstone
Packstone
Matrix support
Grainstone
Packstone
Wackstone Muddy limestone/Micrite
Fig. 5 Carbonate ramp depositional model for the Paleogene Kalatar Formation in SWRTB
3
10
Porosity Plane porosity
8 6 4 2
Permeability, md
Porosity/plane porosity, %
12
2
1
0
0 Outer ramp
Middle ramp
Middle ramp
Outer ramp
Sedimentary subfacies
Sedimentary subfacies
b
a
Fig. 6 Average properties of rocks within different subfacies in the carbonate ramp of the Paleogene Kalatar Formation in the SWRTB. a) Average porosity and plane porosity histogram; b) Average permeability histogram
9
porosity plane porosity
8
2
7
Permeability, md
Porosity/plane porosity, %
10
6 5 4 3 2 1
1
0
0 Calcsparite limestone Micritic-calcsparite limestone
Micritic limestone
Cement types
a
Oolotic dolostone
Calcsparite limestone Micritic-calcsparite limestone
Micritic limestone
Cement types b
Fig. 7 Properties of rocks with different cements in the middle ramp of the Kalatar Formation in the SWRTB. a) porosity and plane porosity histogram; b)permeability histogram
Oolitic dolostone
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1.0 Upper range
0.5 0.4
100
N=75
N=61
0.6
N=61
20th% Lower range
N=40
Vsh, %
0.7
1000
N=40
60th% Mean Median 40th%
N=52
0.8
Deep resistivity, ohm m
80th%
N=75
0.9
0.3 10
N=52
0.2 0.1 0.0
1 Shallow shoal
Deep ramp
Tidal flat
Lagoon
Depositional facies
Shallow shoal
Deep ramp
Tidal flat
Lagoon
Depositional facies
Fig. 8 Box and whiskers plots showing shale volume (Vsh) and deep resistivity of sedimentary microfacies in the Paleogene Kalatar Formation in the SWRTB
sedimentary facies, where facies such as the grain shoal subfacies in the internal tidal ramp and middle ramp are easier to be eroded. Burial leaching can be characterized by a process and phenomenon that rock are subjected to leaching by hydrocarbon fluid and other organic matter in the mediumdeep stage of burial. 6.2.4 Compaction and pressure solution Carbonate sediments must be subjected to compaction in variable degrees before metamorphism. Compaction is a main source of diagenesis responsible for pore reduction, which can make the original porosity of carbonate decrease in the diagenetic process. Presure solution can take place through the process of compaction. Generally, presure solution occurs after calcite cementation and contemporaneous dolomitization in forms of mosaic, deformation, and fracturing between grains. The process can be divided into two kinds of deformation such as ductile and rigidity characterized by different arrangements of deformation and contacting forms between grains (Moore, 2001). Compaction and presure solution are commonly found in the Kalatar Formation in the form of stylolites (Fig. 9D; Fig. 10), which are cemented by mud or organic matter and makes it insignificant in terms of pore development. Reservoirs accommodation, however, can be formed when stress direction is shifted and reversed to some extent in the late burial stage. 6.2.5 Neomorphism Neomorphism can be defined as replacement of one mineral by another in situ. Pores of a dissolved or structural origin can be largely reduced by neomorphism in the late stage, which is not favorable for reservoirs (Moore, 2001). Porosity in reservoirs are generally reduced by pseudosparitization (Fig. 9E; Fig. 10) and pyritization of
Paleogene limestone in the SWRTB (Fig. 9F; Fig. 10). Limestone of the Kalatar Formation is characterized by containing terrigenous quartz grains, which has a great influence on production methods for reservoirs, such as acidification, fracturing, and gas injection. Mixed terrigenous quartz grains in limestone can make acidification more difficult and lead to ineffective fracturing by cementing fractures (Loucks, 1999). Pyrite is an indication of an acidic diagenetic setting. This factor can be reduced by pyrite adhering to bioclastics (Ronchi et al, 2010). 6.2.6 Dolomitization Dolomite mainly occurs in the middle and lower part of the Kalatar Formation on the Maigaiti slope, Kekeya area, and periphery of Qimugeng (Fig. 1). Some intervals of the Kalatar Formation in the Kukebai and Kangsu sections located in front of South Tian mountain are characterized by well developed dolomite with stable horizontal extension. Two dolomitization models with different mechanisms, the evaporative pumping model and brine reflux seepage model, can be established for the Paleogene Kalatar Formation, based on outcrop and core observations as well as laboratory experiments (Fig. 11). Generally, dolomitization is favorable to the forming of hydrocarbon reservoirs with high porosity. Implications for reservoirs based on two kinds of dolomitization models are explained explicitly, as follows. The first, entails dolomitization in a tidal evaporate facies. Dolomites from the supratidal setting are mainly fully dolomitized, laminated fine crystal dolomites as found developing in the Keliyang section, Well Tacan2, Kangsu section, and Kukebai section. These show dolomitized sediments in the supratidal setting under restricted conditions and huge evaporation. The ion ratio of magnesium and
310
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311 Contemporaneous stage
Sequenceof diagenesis event
Fresh waterSubocean setting
Early diagenetic stage
Epidiagenetic stage Epidiagenetic setting
Shallow-medium burial diagenetic setting
Cementation
Micritization and granular cementation
Dolomitization
Erosion and filling Grain point
Compaction Neomorphism and pseudosparite
Hydrocarbon emplacement Gypsum cementation Structural fracture of late stage Fresh water erosion
Stylolite
Replacement Medium scale erosion and enlargment of coarse calcite in interparticles Precipitation of shallow burial calcite Bioclastic fragments and serpulite replaced by silica
Dedolomitization Pyritization
Pore granular calcite cementation
Sutured contact
Erosion
Silicification
Medium-deep burial diagenetic setting
Hydrocarbon emplacement
Fibrous cementation
Micritization
Structural fracture
Late diagenetic stage
Dolomite replaced by calcite Rims of echinoderm fragments replaced by pyrite Fluorescence in intraparticles and interstice of crystallines / bitumen in fissure Filling and cementation along ersion within particles Calcite filling in inter and intra particles and inter crystalline and dedolomitization Enlargement of erosion along the wall of veins and inter and intra particles
Early stage of Miocene
Fig. 10 Evolutional sequence of diagenesis of the Paleogene Kalatar Formation in the SWRTB
pumping and brine reflux seepage (Fig. 10). On the basis of diagenetic imprintI n the Kalatar Formation, diagenetic evolution of outcrop sections has fallen into a epigenetic stage after an early diagenetic epoch due to tectonic uplift (Fig. 11). Well intervals buried at depth are easily subject to continuous subsidence from early diagenesis directly into a setting of medium-deep burial in late stage diagenesis (Fig. 11). Larger amounts of reservoir accommodation are formed during and after a contemporaneous dolomitization period, which leads to a slight modification of erosion in the medium-deep burial setting of early diagenesis. Pore types are dominated by intraparticles, moldic pores, intergrain pores, and occasionally by small tectonic erosional fractures. Reservoir accumulations of relatively small scale are found in core samples. Pervasive hydrocarbon fluorescence found in outcrop sections indicates that the phenomenon of hydrocarbon migration is occurring in the early diagenetic stage and there are sufficient source rocks in relation to reservoirs. It can be inferred that the diagenetic stage is restricted to late-stage diagenesis with intensive compaction. The quality of reservoirs can be largely and greatly improved by structural fractures in late-stage diagenesis (Walkden, 1979). But calcite precipitated from fresh water in the late stage can reduce porosity of a reservoir leading to a detrimental impact on the accommodation and quality of the reservoir (Fig. 11). Reservoir accommodations can be well conserved due to hydrocarbon migration in the early diagenetic stage (Lander and Walderhaug, 1999), which can promote a more successful exploration of favorable entrapments.
6.4 Structural fractures Pore types of reservoirs in the Kalatar Formation in the SWRTB can be divided into a fracture-pore type, pore type, and fracture type, respectively (Fig. 12). There is a high ratio of fractures in the Kalatar Formation, according to statistics and analysis of core samples. The capability of seepage in reservoirs can be greatly improved by fractures and permeability of fractures can surely account for a large share of total permeability in reservoirs (Hooker et al, 2002). Therefore, it is necessary that fracture permeability and regulation of distribution in reservoirs are predicted (Hennings et al, 2000). Most fractures in reservoirs are of structural origin, which is controlled by tectonic stress (Wang, 2004). The degree of fracturing is measured by approaching the coefficient of rock body fractures according to the theory of molar intensity of lithomechanics (Jia et al, 2010). There is some positive correlation between fracture permeability and the approach coefficient of rock body fractures, that is: Kf∞Aη, where Kf is fractures permeability, η is the approach coefficient of rock body fractures, and A is a coefficient. The η can be expressed as the following formula:
K
V1 V 2 V V2 c ( ) sin I 1 tan I 2
where the σ 1 and σ 2 are the maximum and minimum of principal stress respectively, C is the cohesive force of rock, and φ is coefficient of internal friction (Wang, 2004). The formula Kf= 1.23η can be acquired in combination of
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Inner ramp Tide flat
Middle ramp
Outer ramp
Lagoon
Flood line
High energy shoal
Subtidal line
Evaporative pumping dolomitization Brine reflux seepage dolomitization
A
B Keliyang, Qimeigan, Tongyouluke Well Tacan2,Well Keshen103
Kangsu section
Sections or wells Fig. 11 Contemporaneous-penecontemporaneous dolomitization model of the Paleogene Kalatar Formation in the SWRTB A: evaporative pumping model; B: brine reflux seepage model
regression analysis of measured K f on the basis of detail studying of data collected from cores and outcrops in the SWRTB. Computational model of fractures permeability of the Kalatar Formation can be established (Pu and Qing, 2003), which can be used for prediction of distribution of fractures permeability of the Kalatar Formation (Fig. 13).
influence on depositional setting, composition, and diagenesis of sediments, which finally determines the property and characteristics of reservoirs (Fig. 14). Table 2 Comparison of characteristics of reservoirs and diagenesis between different Paleo-climatic conditions in the SWRTB (modified from Shen, 2010)
6.5 Climatic conditions The Eocene Epoch, when the Kalatar Formation was deposited belongs to a greenhouse interval in global climatic variation, which indicates that the climate during that time in the SWRTB is consistent with a hot arid subtropical zone. Diagenesis is universally influenced by climate. Development of karstification is dependent on paleo-climatic conditions having a close relationship with the concentration of carbon dioxide, temperature, and humidity (David, 2005). Diagenesis can be greatly intensified in a warm humid tropical environment, which is favorable to development of reservoirs. On the contrary, diagenesis can be effectively reduced due to an arid hot climate, which is an impediment to reservoirs of high quality (Table 2). Karstification of reservoirs was poorly developed due to the hot and arid tropical and subtropical climate in the middle Eocene of the Kalatar Formation in the SWRTB (Scheibner and Speijer, 2008). A conglomerate layer occurs in the basal part of the Kalatar Formation in the Wubulate and Kalabieledaban sections indicating that there was a rapid climatic cooldown and lowering of sea level. Such a climate and the reduced depth of marine water depth at that time was not favorable for development of carbonate and reservoirs with high quality. Composition and textures of original sediments also play an import role in reservoirs, in addition to variation of sea level corresponding to climate change. Concentration of CO2, temperature, and humidity are greatly affected by variation of climate and have a remarkable
Diagenesis and characteristics of reservoirs
Paleo-climatic conditions Climate of Humid Tropical-Subtropical zone
Climate of arid Subtropical-Temperate zone
Early marine cementation
Local
Common
Dolomitization
Local
Common
Leaching
Common related to surface exposure
Strongly related to hypersaline brine water
Calcite cementation
Common
Rare
Gypsum cementation
Rare
Common
Pore type
Moldic, erosional, microporosity, fractures, Intercrystalline
Interparticle, moldic, intercrystalline, fissure, microporosity
Controls on quality of reservoirs
Location of strata and Sedimentary facies
Sedimentary facies, strata rich in gypsum, erosional fluid of late stage
Pet.Sci.(2011)8:302-315 2.5
of reservoir accommodation. But dense and indissoluble micrite can effectively reduce the permeability of rock. Two stages of calcite cementation and compaction caused a harmful effect on pores in the Kalatar Formation: quality of reservoirs can be improved by leaching and presure solution; porosity in reservoirs can be obviously reduced by pseudosparitization and pyritization; dolomitization can effectively give a better improvement in quality of reservoirs through evaporative pumping and brine reflux seepage (Melim and Scholle, 2002). Structural fractures at the deep burial stage undisputedly enlarge accommodation of reservoirs, whatever the scale. Climatic conditions also play a relatively important role in controlling the quality of reservoirs, which are mostly demonstrated by a contemporaneous diagenetic stage and epigenetic stage for shallow and deep burial diagenesis. However, all are a little subject to paleo-climatic variations, which have a close relationship with formation water and connate water. For carbonate depositional systems, the types and controlling factors favorable to exploration, reservoirs and targets have been explicitly discussed and analyzed above. Reservoirs in the Kalatar Formation belonging to the clastic rock system feature clastics that are mainly situated in the gypsum mudstone interval and coquinoid limestone that occurs among limestone and in a form of transitional state with limestone. Therein, rock types of reservoirs are dominated by limy fine feldspathic litharenite, limy fine lithic sandstone (Lander and Walderhaug, 1999), and a little siltstone. The diagenetic stage has stepped into epidiagenesis after early diagenesis due to uplift, which shows a great similarity with that of carbonate outcrops according
Fracture type
2.0
Fracture-pore type
1.5
logK, md
313
1.0 0.5 0.0
Pore type
-0.5 -1.0 -1.5
0
5
10
15
20
25
30
Porosity, % Fig. 12 Classification of pore types and relationship between porosity and permeability of the Paleogene Kalatar Formation in the SWRTB
7 Discussion on controlling effects of sedimentary facies and diagenesis Two types of depositional models can be established for the Kalatar Formation, which are the shore-shelf sedimentary model and carbonate-ramp model, respectively. The foreshore subfacies is the most favorable target in the shore-shelf model. Properties of reservoirs are better in the middle ramp than the outer ramp, which makes the middle ramp a more favorable exploration target. This conclusion can be further demonstrated by box and whiskers plots (Stacy et al, 2010) to establish shale volume and deep resistivity, respectively. Diagenesis also plays an important role in the quality of the Kalatar Formation, because detailed thin section observations, analysis of diagenesis, and determination of diagenetic evolutional stages in the Kalatar Formation indicate that micritization played a positive role in protection 136˚96’
136˚98’
137˚00’
137˚02’
137˚04’
137˚06’
137˚08’
>8md 41˚56’
41˚56’
7-8md 6-7md 5-6md
41˚54’
1
4-5md
41˚54’ 1
3-4md
1
2-3md 41˚52’
41˚50’
1-2md 0-1md
2
3
4
5
6
78
6 8
5
4
41˚52’
3
7 Ks103
Ks102
4 3 3 7 6 5
Ks1 Ks101
4
41˚50’
3
4
2
41˚48’
8
4 41˚48’
1 41˚46’
41˚46’
136˚96’
136˚98’
137˚00’
137˚02’
137˚04’
137˚06’
137˚08’
Fig. 13 Plane distribution figure of fracture-permeability of the Paleogene Kalatar Formation in the Kekeya area of the SWRTB (modified from Wang, 2004)
Pet.Sci.(2011)8:302-315
Porosity Concentration of Carbon dioxide Temperature Degree of moisture
Decreasing
Increasing
314
Diagenetic setting
Diagenesis
Fresh water-marine diagenetic setting 1)Ffibrous calcite cementation 2)Mmicritization and granular cementation 3)Cementation and erosion
Shallow-medium burial diagenetic setting 1)Replacement 2)Dolomitization 3)Bitumen emplacement 4)Compaction and pressure dissolved
Epidiagenetic setting 1)Eerosion 2)Dedolomitization 3)Calcitic cementation
Medium-deep burial diagenetic setting 1)Compaction and pressure dissolved 2)Erosion 3)Silification and pyritization 4)Hydrocarbon emplacement
Fig. 14 Schematic diagram showing relationships between diagenesis and variation of porosity, concentration of carbon dioxide, temperature, and degree of moisture of the Paleogene Kalatar Formation in the SWRTB
to diagenetic markers of detrital rocks in the Kalatar Formation. Structural fractures and interstitials between particle boundaries are formed because of uplifting, which can be used as migration pathways for secondary leaching. Accommodation of reservoirs is dominated by structuralerosional fractures enlarged by fresh erosion, enlarging pores among interparticles and intragranular erosional pores, and little dolomite moldic pores beneath surface diagenetic environments (Potma et al, 2001). There is a possibility that erosion mainly resulted from organic acid in relatively thick intervals according to indications of hydrocarbon in an early diagenetic stage of the carbonate system. Fracture systems can be established on condition that acidic fluids emplaced during periods of uplift. Pores can be effectively enlarged by these fluids (Sanfrod and Konikow, 1989), which makes industrial-value reservoirs finally mature.
8 Conclusions 1) Main pore types of the Kalatar Formation in the southwestern region of Tarim Basin are represented by moldic pores, vuggy, intercrystalline pore, interparticle pores, fractures, and mudstone microporosity, respectively. Foreshore environments in the shore-shelf depositional model are the most favorable target. Grainstone and dolomitized grainstone are characterized by a high quality in the middle ramp subfacies of the carbonate ramp model. 2) Diagenesis acting through micritization, cementation, compaction, neomorphism, silicification, and pyritization is detrimental for reservoirs, while leaching and dolomitization can give a favorable modification to the quality of reservoirs. The permeability of reservoirs can be greatly improved by fractures, by which the quality of reservoirs can be effectively enhanced. 3) The concentration of CO2, temperature, and humidity are affected by a change of climate, which can have a strong influence on the depositional setting, composition and diagenesis of sediments, and eventually the beneficial properties of reservoirs. The hot and arid climate of the Paleogene was harmful for the development of reservoirs. 4) Pervasive hydrocarbon fluorescence found in outcrop sections indicates that the phenomenon of hydrocarbon
migration took place during the early diagenetic stage and there were sufficient source rocks for reservoir formation in the carbonate depositional system. In the detrital reservoir rock system, acidic fluids emplaced during periods of uplift effectively enlarged pores by which fluids eventually generate reservoirs of industrial value.
Acknowledgements The English text was improved by Prof. Markes Johnson with Williams College, USA. The study was financially supported by the National Natural Sciences Foundation of China (Grant No. 40872078), the Knowledge Innovation Program of South China Sea Institute of Oceanology (Grant No. LYQY200806) and the Research Project of Tarim Oilfield Company (Grant No. 41009080051). We wish to thank Xiao Zhongyao, Huang Zhibin and Liu Yongfu for providing research facilities and conveniences, which enabled this work to be completed.
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