Main Reservoir Space Research Articles

The reservoir characteristics and their significance for deliverability in metamorphic granite buried hill: a case study from the JZS oil field in the Liaodong Bay Basin, NE China

The previous studies show that the fractures are the main reservoir space of metamorphic rocks. But the oil and gas production capacity of the fracture segment is obviously different in JZS oil field. Demarcating the reservoir zones finely and finding the high quality seepage of reservoirs are the keys to explore and develop the metamorphic rock oil field efficiently. Based on the data of cores, thin section, physical property, conventional logging, imaging logging, and production logging, the reservoir types and their logging response characteristics of the JZS oil field in Liaodong Bay Basin were analyzed, and the differences in capacity of different zones of reservoirs are discussed. The results show that (1) the reservoir of the study area can be divided into weathering crust zone, natural fractures zone, and structural breccia zone, and different reservoirs have different logging response characteristics; (2) the weathering crust zone and the structural breccia zone are the main effective seepage reservoirs, and the seepage capacity of the fractured reservoir is greatly changed; and (3) the reservoir type, fracture density, and the relationship between fracture and that of maximum principal stress control the deliverability difference of the metamorphic rock reservoir.

Lithofacies and reservoir characteristics of Permian volcanic rocks in the Sichuan Basin

In order to provide basis for further exploration and deployment of volcanic gas reservoirs in the Sichuan Basin, we classified Permian volcanic rocks in this basin using drilling, mud logging, assay, logging and seismic data comprehensively. Then, the lithological characteristics, lithofacies characteristics, lithofacies distribution and reservoir characteristics of different types of volcanic rocks were studied. Besides, the main factors controlling the development of volcanic reservoirs were determined. And the following research results were obtained. First, volcanic rocks in the Sichuan Basin can be comprehensively classified into 4 categories (i.e., volcanic lava, volcaniclastic lava, volcaniclastic rocks and sedimentary volcaniclastic rocks), and basalt, breccia lava, tuffaceous breccia lava, tuff, sedimentary tuff are mainly developed. Second, 3 types of volcanic lithofacies are extensively developed in the Sichuan Basin, including overflow facies, effusive facies and volcanic sedimentary facies. Overflow facies is mainly dominated by basalt, effusive facies is mainly composed of volcaniclastic lava, and volcanic sedimentary facies is mainly composed of tuff and sedimentary tuff. Third, volcanic reservoirs are lithologically composed of breccia lava and basalt. Breccia lava reservoirs are pore-type reservoirs of moderate porosity and low permeability with devitrified micropores and dissolution pores as main reservoir space. In basalt, fractures and dissolution pores are mainly developed, and they are porous-fractured reservoirs with lower matrix porosity. Fourth, the factors controlling volcanic reservoirs mainly include lithology, lithofacies, later fluid alteration and tectonism. Volcaniclastic lava is devitrified in the process of rapid condensation, and a great number of dispersion-shape micropores are formed to provide favorable conditions for further dissolution and pore enhancement in the later stage. Post-diagenesis effect (e.g. dissolution) and fracture development make the reservoir properties better. In conclusion, the areal distribution of volcanic reservoirs in the Sichuan Basin is mainly controlled by the lithofacies. Porous-fractured basalt reservoirs are mainly developed in the Ya'an–Leshan–Pingshan area in the southwestern Sichuan Basin, and pore-type volcaniclastic lava reservoirs are mainly developed in the Chengdu–Jianyang–Santai area.

Characteristics and dolomitization of the Lower Qiulitage Group reservoir in Bachu Uplift, Tarim Basin, China

The characteristics of the Lower Qiulitage Group in Bachu uplift, which originated in the Upper Cambrian period, are not clear. Based on four core observations, identification of 40 thin sections, and geochemical analysis of samples, this study concludes that crystal dolomite reservoirs developed in the Lower Qiulitage Group in Bachu uplift. Intercrystalline pores and dissolved pores formed the main reservoir space, while dissolution pores and fractures were developed in the middle and bottom part of the formation. The reservoir features low porosity and low permeability, and the fine-medium crystalline dolomite reservoir has the best pore structure. According to a petrochemical analysis, the Lower Qiulitage Group reservoir experienced intense dolomitization, with slightly higher content of trace elements Mn and Sr and negative oxygen isotropy. Tectonic hydrothermal dolomitization developed locally under the influence of base faults, and vug pores were filled with saddle dolomites and siliceous filling. The presence of rare earth elements Ce and Eu presented positive anomaly characteristics. It suggested that the CO2 acid dissolution caused by hydrothermal fluids may be a new mechanism for the increase in pores in deeply buried reservoirs, which is worth further study.

Characteristics and Mechanism of Upper Permian Reef Reservoirs in the Eastern Longgang Area, Northeastern Sichuan Basin, China

Abstract As a typical kind of lithologic reservoirs, reef reservoirs are generally featured by their large single-well reservoir thickness, good reservoir physical properties, and high gas well productivity. The Upper Permian Changxing Formation is an important natural gas exploration target in the Sichuan Basin, which hosts a large reef gas reservoir and is mainly distributed along the Kaijiang-Liangping Trough. Comprehensive analyses are implemented to investigate reservoir characteristics and identify controlling factors of reef reservoirs in Changxing Formation in the Eastern Longgang Area, Northeastern Sichuan Basin, including core, logging, and seismic data analyses. Changxing Formation reservoirs in the study area mainly occur in the reef-shoal complex, which are featured by wide distribution, large thickness and generally good physical properties. Reservoir rocks are dominantly composed of bioclastic dolomite and silty-fine dolomite (with grain phantom structure), while the main reservoir space consists of residual intergranular pores, intergranular dissolution pores, and karst vugs. In the seismic profiles, typical mound-shaped chaotic reflections can be clearly seen. It is suggested by the main reservoir development controlling factor analysis that the distributions of reef reservoirs are typically controlled by sedimentary facies belts, while the scale of the reef-shoal complex is determined by the pene-sedimentary micro-paleo-geomorphology. Dolomitization can not only significantly preserve the primary pores but also enhance the permeability of rocks. Moreover, karstification is the key to high-quality reef-shoal reservoirs.

The forming mechanism of high quality glutenite reservoirs in Baikouquan formation at the Eastern slope of Mahu sag of the Junggar basin, China

By combining the cores, thin sections, physical properties, and the mercury injection curves, forming mechanism of high quality glutenite reservoirs in Baikouquan Formation at the eastern slope of Mahu Sag is studied. It is a fan-delta sedimentary system of coarse debris gravity flow deposits, with glutenite as the dominant reservoir lithology and dissolution pores as the main reservoir space. High quality reservoirs is primarily controlled by the sedimentary facies and diagenesis. The fan-delta front facies provides material basis, while the dissolution during diagenesis constitutes the primary development mechanism.

Geochemistry of the fracture-cave calcite of the Ordovician Yingshan Formation and its paleokarst environment in the Tazhong area, Tarim Basin, China

The fracture-cave reservoir of the Yingshan Formation is an important oil and gas producing zone in the Tarim Basin. Karstification occurs at the top surface of the carbonate rocks of the Yingshan Formation on the northern slope of the Tazhong area due to a short exposure, so abundant fracture-cave spaces are formed. On the basis of the data of nearly 49 samples obtained from 21 wells, the carbon and oxygen isotopes of the fracture-cave calcite were analyzed. The carbon and oxygen stable isotopic characteristics recorded during paleokarstification were discussed to provide geochemical evidence for predicting the distribution of the fracture-cave reservoirs, as well as to understand the genetic mechanism of karst fracture-caves and the fluid properties and paleo-hydrological conditions of paleokarstification. The fracture-cave calcite can be divided into three types based on its carbon and oxygen isotope values, which indicate three types of paleokarst environments with different fluid properties. Type I calcite has similar carbon and oxygen isotope values to the bedrock of the Yingshan Formation, with δ13C values of − 2.0 to 1.1‰ and δ18O values of − 5.76 to − 8.16‰. These values indicate that the calcite was precipitated in the eogenetic karst environment of the Yingshan Formation. Type II calcite has a δ13C value of − 2.13 to 1.0‰ and a δ18O value of − 12.41 to − 8.0‰. It has a similar δ13C value, but a significantly negative oxygen isotope value compared with values of the bedrock of the Yingshan Formation, indicating a buried karst environment. Type III calcite has an extremely negative δ18O value (δ18O value < − 14.50‰), indicating hypothermal karst genesis. Its formation is related to magma or deep hydrothermal fluids. No calcite with atmospheric freshwater genesis and significantly negative δ13C and δ18O values was found in the samples, indicating that the formation of fracture-caves is weakly affected by atmospheric freshwater. The main reservoir space of the Yingshan Formation is karst caves. The calcite in the karst cave fillings belongs to type I, indicating an eogenetic karstification environment. Therefore, the formation of the fracture-cave reservoirs of the Yingshan Formation in the Tazhong area is related to eogenetic karstification, which occurred during exposure and mainly contributed to the formation of the reservoir spaces.

Origin of crystal dolomite and its reservoir formation mechanism in the Xixiangchi Formation, Upper Cambrian in Southeastern Sichuan basin

This paper takes the thick and widely distributed crystal dolomite of Xixiangchi Formation, Upper Cambrian in Southeastern Sichuan basin as an example, the reservoir characteristics and main controlling factors of crystal dolomite have been analyzed systematically, and the genesis and reservoir formation mechanism of crystal dolomite have been discussed. It shows that the main reservoir rocks of crystal dolomite include powder crystalline dolomite and finely crystalline dolomites, and the intercrystal pores and dissolved intercrystal pores are the main reservoir spaces, with few dissolved vugs and fractures developed in the reservoir. The pore structure analysis shows that the throats of crystal dolomite reservoir are mainly lamellar, reflecting a good configuration between pores and throats, medium- low porosity and low permeability, representing typical porous-type reservoir. Dolomitization analysis indicates that the dolomitization occurred in the penecontemporaneous or early diagenetic stage. The degree of dolomitization decreased gradually from the top to the bottom in each sedimentary cycle, and distribution of dolomite in the study area shows that the content of dolomites decreases from the west to the east, which reflects the degree of dolomitization weakening eastward. Combined with the geochemical research of crystal dolomite, the C and O isotope data reflect the same characteristics of seawater during the late Cambrian. It indicates that the dolomitization might be related to the seepage-reflux of seawater in the early time. Recrystallization played an important role in the genesis of the crystal dolomite, because most of crystal dolomites remain residual granular texture. The intercrystal pores formed by seepage–reflux dolomitization acted as superior seepage channels for the reservoir transformation, and the recrystallization adjusted the previous pores and made them better, while the karstification was the key to the dissolved pore formation.

Reservoir characteristics and genetic differences between the second and fourth members of Sinian Dengying Formation in northern Sichuan Basin and its surrounding areas

Abstract This study investigated the characteristics and genesis of reservoirs in the 2nd and 4th members of Sinian Dengying Formation in northern Sichuan and its surrounding areas, on the basis of outcrop, drilling cores and thin section observation and geochemical analysis. The reservoirs of 2nd member are distributed in the middle part of the stratum. The reservoir quality is controlled by supergene karst and the distribution of mound-shoal complex. The bedded elongated isolated algal framework solution-cave and the residual “grape-lace” cave, which are partially filled with multi-stage dolosparite, constituted the main reservoir space of the 2nd member. There is no asphalt distribution in the pores. The pore connectivity is poor, and the porosity and permeability of the reservoir is relatively low. The reservoirs of 4th member were distributed in the upper and top part of the stratum. The reservoir quality is controlled by burial dissolution and the distribution of mound-shoal complex. The bedded algal framework solution-pores or caves, intercrystalline pores and intercrystalline dissolved pores constituted the main reservoir space of the 4th member. It's partially filled with asphalt and quartz, without any dolosparite fillings in the pores and caves. The pore connectivity is good. Most of the 4th member reservoirs had medium-low porosity and low permeability, and, locally, medium-high porosity and medium permeability. Affected by the development of mound-shoal complex and heterogeneous dissolution, the platform margin along Ningqiang, Guangyuan, Jiange and Langzhong is the most favorable region for reservoir development. Deep buried Dengying Formation in the guangyuan and langzhong areas should be the most important hydrocarbon target for the future exploration.

Reservoir controlling factors of the Paleogene Oligocene Huagang Formation in the north central part of the Xihu Depression, East China Sea Basin, China

Huagang Formation is a typical offshore, deep, low-permeability, tight sandstone reservoir with a low level of exploration and development and unclear control factors in the North Central area of the Xihu Depression in the East China Sea Basin. In this paper, we begin with the analysis of basic data, such as cores, casting thin sections, cathode luminescence, scanning electron microscopy, physical property statistics, grain size analysis, mercury penetration and magnetic resonance imaging, combined with field logging data from conventional logging, imaging logging and three-dimensional seismic imagery. The reservoir's physical properties were comprehensively discussed in terms of provenance, deposition, diagenesis and fractures. The results showed that the reservoir physical properties of the Huagang Formation were mainly affected by compaction, and the porosity and permeability decreased with the depth of burial. When the depth exceeds 4000 m, the porosity is basically <10% and the permeability is nearly <1 mD. The compaction above 4000 m was mainly controlled by provenance and depositional microfacies. The reservoir physical properties in the northern provenance is better than that of the east provenance. A high-energy braided channel is the best physical depositional microfacies. The reservoir formed by event sedimentation, such as from storms and floods, due to poor sorting, often containing mud and so on, with poor physical properties. Carbonate cementation only had a great influence on the local reservoir. Illite, illite/smectite and pore-filling chlorite were important factors affecting physical property deterioration. Pore-lining chlorite can promote the retention of primary pores. Below 4000 m, favorable reservoirs were mainly controlled by dissolution, resulting in dissolution pores being the main reservoir space. Microfractures played an important role in the properties of low-permeability and tight reservoirs. Microfractures themselves were easily dissolved into dissolution pores while being an important seepage channel for low-permeability and tight reservoirs. The denser the reservoir is, the more important the microfracture seepage is.

Genetic types of carbonate shoal reservoirs in the Middle Triassic of the Sichuan Basin (SW China)

Abstract Carbonate shoal reservoirs of the Middle Triassic Leikoupo Formation in the Sichuan Basin were investigated for potential gas resources. The carbonate shoal reservoirs can be divided on the basis of their formation mechanism into residual intergranular pore, karst (further divided into syndepositional-, burial-, and epigenetic-karstification types), dolomitized, and composite reservoirs. Residual intergranular pores and dissolved pores are the main reservoir spaces in the residual intergranular pore reservoirs. Isolated intragranular corroded pores and moldic pores are the main reservoir spaces in syndepositional-karstification reservoirs, the main spaces in burial-karstification reservoirs are dissolved pores and holes, and pores and caves on various scales are the main reservoir spaces of epigenetic-karstification reservoirs. Intercrystal pores and intercrystal solution pores are the main reservoir spaces of dolomitized reservoirs. Composite reservoirs are characterized by a variety of reservoir spaces with pores, holes, and fissures. High-quality shoal reservoirs were formed by superposition of carbonate shoals and constructive diagenetic modification. Preservation of residual intergranular pores was crucial for development of residual intergranular pore reservoirs. Platform-interior micro-highlands, relative declines in sea level, and rainfall controlled the development of syndepositional-karstification reservoirs. Dissolution by unsaturated acidic fluids, pre-existing pores and pore throats, and the direction of corrosive fluid flow were important for burial-karstification reservoirs. Epigenetic-karstification reservoirs formed as a result of significant sea-level drops or tectonic movement. Dolomitized reservoirs formed in shoals affected by dolomitization caused by evaporation of seawater or that occurred during burial. Composite shoal reservoirs were formed by two or more processes.

Volcanic reservoir characteristics and hydrocarbon genesis of Jiamuhe formation in Jinlong 2 wellblock, Junggar Basin

Due to deeper buried, multi-stage tectonic influences, rapid changing, and unclear originate of oil and gas in Jinlong 2 wellblock, we analyzed the volcanic reservoir characteristics and the genesis of hydrocarbon by cores, thin slices, loggings, physical properties, oil testing data, and carbon isotopes. Results demonstrated that the major lithology were pyroclastic rocks and lavas, the reservoir was generally poor and strongly homogeneous, the main reservoir space were primary pores, and the favorable reservoir mainly developed in the pyroclastic rocks of the third phase. Furthermore, the results of carbon isotopes indicated that the crude oil and dissolved gas originated from Fengcheng formation, while the condensate gas derived from Jiamuhe formation.

Diagenetic evolution characteristics of Paleogene lacustrine carbonate reservoirs in central Bohai sea, China

The distribution of high-quality reservoirs in Shahejie Formation of the Shijiutuo Uplift, Central Bohai Sea, is the key factor to determine the oil and gas accumulation. Likewise, diagenesis is the key factor to control the development of lacustrine carbonate reservoirs in Shahejie Formation. So, accurately analyzing the diagenesis is very important for improving reservoir prediction. By means of polarizing microscopy, fluorescence microscopy, scanning electron microscopy, cathodoluminescence and electron probe analysis techniques, reservoir characteristics and diagenetic evolution sequence are determined. The diagenetic stage was determined by X-ray diffraction of clay minerals. Gas-liquid inclusion homogenization temperature and oxygen isotope geothermometer are used to analyze the formation period of authigenic carbonate minerals. Bioclastic dolostone is the main type of carbonate rocks, followed by dolarenite. The visceral cavity pores and residual primary intergranular pores are the main reservoir space. The diagenetic evolution sequence is established based on the different occurrences of multistage dolomites and their relationship with other diagenesis phenomena, and the final diagenetic stage is the phase B of mesodiagenesis. The favorable diagenesis for reservoir physical properties is the early formation of the pore-lining dolomite deposited in the eodiagenesis and the grain-coating dolomite deposited in the syndiagenesis, both of which weaken the subsequent mechanical compaction to the reservoir porosity.

Evaluation of dolomite reservoirs in the Longwangmiao Formation, Lower Cambrian in Northern Sichuan basin, China

Abstract With the extended petroleum exploration of the carbonate reservoir in the Longwangmiao Formation, Lower Cambrian in Sichuan basin, looking for a new favorable exploration area is the primary task in current exploration. This paper focusing on the dolomite reservoir in the Longwangmiao Formation in the Northern Sichuan basin, will evaluate the reservoir conditions, including reservoir lithology, reservoir spaces, physical properties and other basic characteristics. Combined with sedimentary environment study and diagenesis analysis, the genetic mechanism of Longwangmiao reservoir in study area will be discussed. It indicates that the reservoir rocks of Longwangmiao Formation in the study area are composed of dolarenite and crystalline dolomite, and the main reservoir spaces are intergranular dissolved pores, intragranular dissolved pores and intercrystalline dissolved pores. However, most of dissolved pores have been filled with bitumen, which lead to the reservoir properties very poor, both the porosity and permeability are worse than that of central Sichuan basin, so the reservoir in the Longwangmiao Formation in the Northern Sichuan basin is low-porosity and low-permeability reservoir. During the Early Cambrian Longwangmiao period, the study was near the old-land, and the energy of sea water was not enough to deposit many grain shoals, which cause the sediments are small with lots of quartz and muds. Moreover, because lacking of large scale karstification in the late diagenesis, there isn't much dissolved pores and vugs develop in the Longwangmiao Formation. Some Intergranular dissolved pores and intercrystalline dissolved pores formed by the early atmosphere water dissolved exist as some reservoir spaces, but most of them have been filled with bitumen, which was residual when the ancient oil reservoir transformed. Above factors cause the Longwangmiao Formation in the Northern Sichuan basin become not a very favorable reservoir in Sichuan basin.

Geological characteristics and main controlling factors of Permian lacustrine tight oil in the eastern part of the Junggar Basin

The Junggar basin is rich in oil and gas resources, and recently, unconventional oil and gas reservoirs have become important targets. Significant amounts of oil and gas are located in Permian formations in the Shazhang–Dajing area, however, fewer studies have focused on unconventional oil and gas reservoirs, particularly on the factors controlling the distribution of unconventional reservoirs. In this study, a sequence stratigraphic framework of the study area was developed based on cores, well logs and seismic data. Source rock samples were analyzed, including tests of kerogen, vitrinite reflectance, chloroform bitumen, organic carbon and group composition. The physical properties and pore characteristics of reservoir samples were also analyzed using scanning electron microscope and high resolution electron microscope. The Permian Pingdiquan Formation comprises three thirdorder sequences in which individual sequence can be divided into a low-stand system tract (LST), a transgression system tract (TST), an early high-stand system tract (E-HST), and a late high-stand system tract (L-HST). Several source rock lithologies have high total organic carbon (TOC), and are matured, having excellent hydrocarbon generation potential. Clastic and dolomitic rocks are the main reservoirs, and nanometer-scale pores are the main reservoir space in the tight reservoirs. The distribution of tight oil is controlled by the sequence stratigraphy, palaeogeomorphic units, and sedimentary facies. The fine sediments, which were generally deposited near the initial flooding surface (FFS) and the maximum flooding surface (MFS), are favorable zones for tight oil. The distributions of the hydrocarbon source rocks are controlled by the depositional environment, depending on the palaeogeomorphic units; the deep depression in the Huoshaoshan area and the moderately-deep depression in the Shishugou area are two distinct hydrocarbon generating centers. The distribution of tight oil is clearly controlled by the sedimentary facies; the tight oil is mainly located in the fan delta front and lacustrine depositional environments. The distribution of tight oil in this area is summarized, and favorable areas of tight oil are proposed.

Geological characteristics of source rock and reservoir of tight oil and its accumulation mechanism: A case study of Permian Lucaogou Formation in Jimusar sag, Junggar Basin

Abstract The source rocks and reservoirs in the Middle Permian Lucaogou Formation of Jimusar sag, Junggar Basin, NW China and its source-reservoir assemblages were studied to reveal accumulation mechanism of the tight oil, on the basis of core observation, thin slice identification, X-ray diffraction analysis, TOC test and rock pyrolysis test of source rock samples, and mercury injection and nuclear magnetic resonance of reservoir samples, combined with burial history and hydrocarbon-generating history of source rocks. It is concluded that the Lucaogou Formation source rocks mainly have type I and II organic matter, with an average TOC of 4.6%, indicating they are high-quality source rocks in low mature – mature stage; and they can be divided into three types, shale, mudstone and silty mudstone. The reservoir rock in Lucaogou Formation features low porosity and low permeability; mercury injection and NMR (nuclear magnetic resonance) tests show that nano-scale pores and throats (with radius of 50−500 nm) constitute the main reservoir space, and the reservoirs can be classified into three types, muddy siltstone, dolomitic siltstone, and dolarenite according to lithology. The source rocks and reservoirs alternate with each other in vertical direction and are distributed continuously in lateral direction, and the source rocks have been generating oil and gas constantly. It is concluded through comprehensive analysis that since the Late Jurassic, the source rocks in Lucaogou Formation have been generating oil continuously and massively, causing high pressurization, and providing constant driving force and oil supply, as a result, the oil has been charged into the tight reservoirs effectively and continuously, forming abundant tight oil resources.

Shale gas enrichment pattern and exploration significance of Well WuXi-2 in northeast Chongqing, NE Sichuan Basin

Abstract The shale gas enrichment pattern of Well Wuxi-2 in northeast Chongqing was studied, based on the data of the drilling, graptolite biostratigraphy, geochemistry, rock minerals, microscopic characteristics of reservoir beds and tectonic conditions, etc. The organic-rich shale of Upper Ordovician Wufeng Formation—Lower Silurian Lungmachi Formation is 89.8 m thick in Well WX-2. The graptolite biozonations are completely developed in this well, and the organic-rich shale intervals extend upward from the late Katian of the Ordovician to the early Telychian of the Lower Silurian. The deposition time of the organic-rich shale is far longer and the thickness is larger than those in other areas of the Sichuan Basin. The highest measured gas content exceeded 8 m 3 /t in Well WX-2, and the gas content is mainly controlled by TOC. The organic nano-pores are the main storage space, and the minerals contribute less to the storage space. The organic pores larger than 50 nm are well-developed and those less than 10 nm are the main reservoir space of adsorbed gas. The target intervals of Well WX-2 are located under the neutral surface of compressional Tianba anticline. Two vertical fracture (cleavage) development zones, which are beneficial for shale gas storage and complex fracture network formation during later fracturing, were formed in brittle layers of this organic-rich shale. Compressional faults existed in two limbs of the Tianba anticline, with non-permeable shale developing on both sides of fault planes and development of clay smear, which shows that the faults have good sealing properties, and are favorable for shale gas preservation. Thus the good match between the above various accumulation conditions forms the “tectonic sweet-spot” of shale gas in this study area.

A unique lacustrine mixed dolomitic-clastic sequence for tight oil reservoir within the middle Permian Lucaogou Formation of the Junggar Basin, NW China: Reservoir characteristics and origin

The middle Permian Lucaogou Formation in the Jimusaer Sag of the southeastern Junggar Basin, NW China, was the site of a recent discovery of a giant tight oil reservoir. This reservoir is unusual as it is hosted by lacustrine mixed dolomitic-clastic rocks, significantly differing from other tight reservoirs that are generally hosted by marine/lacustrine siliciclastic–calcitic sequences. Here, we improve our understanding of this relatively new type of tight oil reservoir by presenting the results of a preliminarily investigation into the basic characteristics and origin of this reservoir using field, petrological, geophysical (including seismic and logging), and geochemical data. Field and well core observations indicate that the Lucaogou Formation is a sequence of mixed carbonate (mainly dolomites) and terrigenous clastic (mainly feldspars) sediments that were deposited in a highly saline environment. The formation is divided into upper and lower cycles based on lithological variations between coarse- and fine-grained rocks; in particular, dolomites and siltstones are interbedded with organic-rich mudstones in the lower part of each cycle, whereas the upper part of each cycle contains few dolomites and siltstones. Tight oil accumulations are generally present in the lower part of each cycle, and dolomites and dolomite-bearing rocks are the main reservoir rocks in these cycles, including sandy dolomite, dolarenite, dolomicrite, and a few dolomitic siltstones. Optical microscope, back scattered electron, and scanning electron microscope imaging indicate that the main oil reservoir spaces are secondary pores that were generated by the dissolution of clastics and dolomite by highly acidic and corrosive hydrocarbon-related fluids.

The influences of sea-level changes on the quality of bank reservoirs of the Lower Cambrian Longwangmiao Formation, in the Gaoshiti-Moxi area, Sichuan Province, China

Reservoir properties of banks of the Longwangmiao Formation show strong lateral and vertical variations in the Gaoshiti-Moxi area, central Sichuan. In this study, we utilized outcrop, core data, thin sections, wireline logs, and seismic data to investigate the influences of sea-level changes on the development of banks and penecontemporaneous dissolution which controlled the formation of high-quality bank reservoirs. Our results revealed that the Longwangmiao Formation corresponds to a third-order sea-level change cycle, which consisted of a rapid sea-level rise and a slower sea-level fall; we further identify four fourth-order sea-level cycles. Bank reservoirs are dominated by grain dolostone, silt to fine sand-sized crystalline dolostone, and mottled silt to fine sand-sized crystalline dolostone. The main reservoir spaces were pores and vugs, with an average porosity of 4.81% and average permeability of 4.75 × 10−3 μm2. Bank deposition and dissolution during the third-order sea-level fall developed better than during the rising period, and the late fall stage was better than the early fall stage. Under the influence of fourth-order sea-level changes within the third-order sea-level change, the development of banks shows strong variations between different areas, which are evidenced by the fact that banks in the Moxi block were better developed than those in the Gaoshiti block. Bank dissolution in the Moxi block developed during the early and the late stage within the third-order sea-level fall, whereas the Gaoshiti block was developed only during the late stage. Therefore, we have concluded that sea-level changes played a significant role in controlling the quality of bank reservoirs in the Gaoshiti-Moxi area.

Petroleum geology of Carboniferous volcanic weathered crust in northern Xinjiang, China

Abstract Previous studies on volcanic rock reservoirs were focused on lithology, lithofacies and fractures. Primary processes may lead to high porosity and permeability, and secondary processes tend to decrease primary porosity. However, this is not the case in northern Xinjiang, where Carboniferous volcanic rocks covered an area of 378×103 km2 dominated by intermediate-basic rocks of low porosity. At the end of the Carboniferous, these rocks were uplifted, underwent denudation and weathering, and then formed a weathered crust. All types of volcanic rocks might form favorable reservoirs after long-term weathering. The reservoir properties of the crust were controlled by weathering degree and fault development. Four types of reservoir porosity developed: dissolutional pore, pore-fracture, fracture and fracture-cave. Dissolutional pores and fractures are the main reservoir spaces. Under the influence of fractures and an enhanced fracture dissolution environment, the volcanic reservoirs have improved physical properties in deeper burial settings. The depth range of effective reservoirs under the unconformity, either close to, or far from fracture zones, is between 1100 m and 550 m, with maximum porosities of 32% and 24%, respectively. Hydrocarbons have accumulated close to the effective source rock zone due to strong heterogeneities of the volcanic reservoir. The overlying Carboniferous mudstone formed effective caprock. The current structural highs and slope zone coincided well with the paleogeomorphology and form traps for hydrocarbons. Faults and fractures control hydrocarbon enrichment. This is contrary to the view that the Carboniferous is simply impermeable basement in this region, and not capable of hydrocarbon generation.

厚层灰岩油藏岩溶纵向分带研究

Abstract In district 4 of the Tahe Oilfield, the Ordovician Yingshan Formation was most petroliferous. Marine carbonate rocks with big thickness were widespread. Secondary pores, including dissolution pores, fractures and caves, served as the main reservoir spaces, while primary pores were poorly developed. According to modern karst hydraulic unit theory, a conceptual model of karst was established. Three palaeo-hydraulic zones were identified:vadose zone (including infiltration sub-zone and percolation sub-zone), phreatic zone and tranquil zone. The distribution of reservoirs was described using stochastic seismic inversion technology. The reservoirs were controlled by palaeo-hydraulic zones vertically and karst landforms horizontally, which was proved by field work.