Yingshan Formation Research Articles

Dynamic simulation of differential accumulation history of deep marine oil and gas in superimposed basin: A case study of Lower Paleozoic petroleum system of Tahe Oilfield, Tarim Basin, NW China

According to the complex differential accumulation history of deep marine oil and gas in superposition basins, the Lower Paleozoic petroleum system in Tahe Oilfield of Tarim Basin is selected as a typical case, and the process of hydrocarbon generation and expulsion, migration and accumulation, adjustment and transformation of deep oil and gas is restored by means of reservoine-forming dynamics simulation. The thermal evolution history of the Lower Cambrian source rocks in Tahe Oilfield reflects the obvious differences in hydrocarbon generation and expulsion process and intensity in different tectonic zones, which is the main reason controlling the differences in deep oil and gas phases. The complex transport system composed of strike-slip fault and unconformity, etc. controlled early migration and accumulation and late adjustment of deep oil and gas, while the Middle Cambrian gypsum-salt rock in inner carbonate platform prevented vertical migration and accumulation of deep oil and gas, resulting in an obvious “fault-controlled” feature of deep oil and gas, in which the low potential area superimposed by the NE-strike-slip fault zone and deep oil and gas migration was conducive to accumulation, and it is mainly beaded along the strike-slip fault zone in the northeast direction. The dynamic simulation of reservoir formation reveals that the spatio-temporal configuration of “source-fault-fracture-gypsum-preservation” controls the differential accumulation of deep oil and gas in Tahe Oilfield. The Ordovician has experienced the accumulation history of multiple periods of charging, vertical migration and accumulation, and lateral adjustment and transformation, and deep oil and gas have always been in the dynamic equilibrium of migration, accumulation and escape. The statistics of residual oil and gas show that the deep stratum of Tahe Oilfield still has exploration and development potential in the Ordovician Yingshan Formation and Penglaiba Formation, and the Middle and Upper Cambrian ultra-deep stratum has a certain oil and gas resource prospect. This study provides a reference for the dynamic quantitative evaluation of deep oil and gas in the Tarim Basin, and also provides a reference for the study of reservoir formation and evolution in carbonate reservoir of paleo-craton basin.

The origin of Ordovician siliceous rocks and their deposition settings in the Northwest Tarim Basin, NW China

The siliceous rocks are well developed in Northwest Tarim Basin, which plays an important role in indicating its forming environment. In order to determine the origin of siliceous rock and its deposition settings, the paper studies the petrographic and geochemical characteristics of siliceous rock in well KP 1 and outcrops in Northwest Tarim Basin based on core observation, microscopic section identification and analysis of major and trace elements and rare earth elements. The Ordovician siliceous rocks of Northwest Tarim Basin are mainly developed in the middle and upper Penglaiba Formation and the bottom of the Yingshan Formation. Three types of cherts are identified, including cryptocrystalline silica, microcrystalline quartz and fibrous chalcedony depending on the mineral morphology. The terrigenous source is best evidenced by the high Ti (160∼310 ppm) and Al (1300∼3180 ppm), and positive δ30Si (0.9–3.6‰). Whereas, hydrothermal processes can be inferred from relative high content of V, Ni, Sr, Mo and Ba, and no obvious Ce and Eu anomaly. The chert of well KP 1 and Kuluxi profile is deposited in continental-margin settings, which is evidenced by its Al2O3/(Al2O3 + Fe2O3) (0.58–0.94, average of 0.81) and its Lan/Cen ratio (0.76–1.02, average of 0.93). The continental-margin settings are also evidenced by the close relation between silicification and dolomitization. Silicification is commonly well developed where dolomitization occurs, because the dolomitization mainly occurs in the regression background, where provides more terrigenous source silica to the sea water and results in more silicification. This study highlights the origin of Ordovician siliceous rocks and the outcome from this new contribution may provide useful guidelines for hydrocarbon exploration in the basin and other global analogues.

Facies and geochemical characteristics of the Middle-Lower Ordovician Yingshan Formation in the Tarim Basin, NW China: Implications for the high-frequency sequence stratigraphy in shallow-water carbonate platform

Shallow-water carbonates from the Yingshan Formation are important target of hydrocarbon exploration in Tarim Basin. Detailed descriptions of outcrop, core, and thin sections, isotopic composition analysis, and trace element analysis were conducted. Seven lithofacies which consists of two facies belt groups were identified and used to interpret deposition environments of Yingshan Formation: (1) peritidal carbonate, with relatively lower δ13C values from −4.2% to −1.9%, mainly represented by intertidal to supratidal facies and restricted subtidal facies and (2) open-marine subtidal carbonate, with higher δ13C values (−1.5% to −0.3%), mainly consists of shoal facies and interbank sea facies. On the basis of the lithology sets, four types of meter-scale cycle model (types A–D) were grouped into peritidal sequences and subtidal sequences. On the basis of vertical lithofacies, cycle stacking patterns, and accommodation variations in Fischer plots, two third-order depositional sequences (SQ1–SQ2) were recognized. The sequence boundary between SQ1 and SQ2 is not only a surface of a positive shift of δ13C values, but also the transitional zone of cycle stacking patterns. The sequences can be further divided into four fourth-order sequence sets: Sq1–Sq4. The lower sequences (Sq1–Sq2) are dominated by peritidal facies and characterized by a progressive decreasing in accommodation space probably indicates a longer-term fall in sea level. The upper sequences (Sq3–Sq4) are mainly dominated by subtidal facies and characterized by the accommodation space with a progressive increasement, likely indicates a rising sea level with a longer term. The facies-controlled reservoirs are mainly present in the Sq3, related to the sequence boundary between SQ1 and SQ2.

Origin of gas condensate reservoir in Fuman Oilfield, Tarim Basin, NW China

To understand the reservoir property and hydrocarbon accumulation conditions of the Middle and Upper Ordovician intraplatform shoal between ultra-deep main strike-slip faults in Fuman Oilfield of the Tarim Basin, China, the main strike-slip faults in and around well FD1 in the basin were analyzed in terms of sedimentary facies, sequence stratigraphy, intraplatform shoal reservoir property, and oil and gas origins, based on drilling data. The Yingshan Formation intraplatform shoal between the main strike-slip faults is superimposed with low-order faults to form a new type of hydrocarbon play. Firstly, hydrocarbons generated from the Lower Cambrian Yuertusi Formation source rocks vertically migrated into the second member of Yingshan Formation through the main strike-slip faults, and then migrated laterally until they were accumulated. A small amount of oil from Well FD1 came from the Yuertusi Formation source rocks in the mature stage, and a large amount of gas originated from oil cracking in the ultra-deep reservoirs. Therefore, the secondary gas condensate reservoir in Well FD1 is characterized by high gas to oil ratio, dry gas (dryness coefficient being 0.970) and hybrid origin. This new type of hydrocarbon play characterized by intraplatform shoal and low-order fault suggests a prospect of continuous hydrocarbon-bearing area in Fuman Oilfield, which will expand the ultrap-deep oil and gas exploration in the oilfield.

Rock physics characteristics and their control factors of carbonate in different sedimentary microfacies of the Yingshan Formation, Gucheng Area, Tarim Basin

Abstract Understanding the influence of geological characteristics on rock physics properties is crucial for accurately recognizing the relationship between rock physics variation and reservoir characteristics. Unlike the conventional rock species, the rock physics properties of the deep carbonate rocks in the third member of Yingshan Formation (Ying-III Member) in Gucheng area, Tarim Basin are relatively more complex. To address this problem, we investigated the rock physics characteristics and controlling factors of different sedimentary microfacies samples, combined with sedimentological analysis and rock physics experiments. The results show that the sedimentary environment affects the lithology and pore structure by controlling the properties of the primitive rock and early diagenesis. Dolomitized shoal microfacies and shoal top dolomitic flat microfacies primarily form crystalline dolomite and siliceous dolomite, with pores consisting of inter-crystalline pores, dissolution pores, and cracks. Inter-shoal dolomitic flat microfacies develops silty dolomite, with only a few inter-crystalline pores and cracks. Middle- to high-energy shoal microfacies and inter-shoal sea microfacies develop tight calcarenite and micritic limestone. Samples with similar mineral composition have relatively consistent density values and acoustic properties. Soft pores, such as micro-cracks, have a significant impact on the effective pressure and acoustic wave velocity, velocity and velocity ratio, and velocity and porosity relationships. The research shows a new approach for the rock physics characteristics of deep carbonate reservoirs under geological background constraints, as well as the rock physics basis for seismic prediction of the Ying-III Member reservoir.

Hydrocarbon charging process of Ordovician strike‐slip fault zones in the Shunbei area, Tarim Basin, NW China

Ultra‐deep marine carbonate reservoirs is an important field for oil and gas exploration, and it is also a hot research topic in recent years. However, their hydrocarbon episodic charging model is still under debate up to now. A great breakthrough has been made in the newly discovered ultra‐deep fault‐karst oil reservoir in the Shunbei area of Tarim Basin. Based on quantitative gas chromatogram‐mass spectrometry, stable carbon isotopes of reservoir oils, microthermometry and confocal laser scanning analyses of fluid inclusions, the reservoir oil origin and hydrocarbon charging history in Shunbei No. 1, No. 5 and No. 7 strike‐slip fault zones are expounded. The results showed that the Ordovician reservoir oil derives from the source rocks within the Cambrian Yuertusi Formation deposited in a highly anoxic bottom water environment. There were four stages of hydrocarbon charging in the study area, in the late Caledonian, late Hercynian–Indosinian, middle Yanshanian and late Himalayan. During this period, the force‐field strength of the charged oil gradually decreased, and the early‐stage charged crude oil with low maturity and high force‐field strength preferentially accumulated in the Yijianfang Formation, while the late‐stage charged crude oil with high maturity and low force‐field strength preferentially accumulated in the Yingshan Formation. This study reveals the history and differences of petroleum accumulation in the Shunbei fault zones, which is significance for understanding the hydrocarbon distribution in ultra‐deep marine carbonate reservoirs.

Fault-karst systems in the deep Ordovician carbonate reservoirs in the Yingshan Formation of Tahe Oilfield Tarim Basin, China

Fault-karst systems in deep and ultra-deep carbonate reservoirs have a huge potential for hydrocarbon exploration. However the characteristics of reservoir space and the main controlling factors of fault-karst reservoirs are unclear due to strong heterogeneity. Therefore, the exploration and development of deep and ultra-deep carbonate reservoirs are restricted. To fill the gap, core, thin section and well log (conventional logs and image logs) data are used to analyze the lithology and main reservoir space in the deep Ordovician reservoirs in the Tahe Oilfield, Tarim Basin, China. The lithologies are dominated by intraclastic limestone, dolomitic limestone and silt to fine-sized crystalline dolostone. The reservoir space mainly includes dissolution vug, fracture, fracture-vug and cave. Among them, caves and their surrounding fractures are the main storage spaces for hydrocarbons in the Ordovician reservoir. Four fault-karst systems are divided including “V” shape, “U” shape, “layer” shape and compound type according to the morphology of the seismic section. In addition, the reservoir space of different fault-karst systems is clarified using image logs. Combined with drilling data, it is found that the reservoir scale of “U” shape fault-karst system is larger, while that of “layer” shape fault-karst system is smaller. In addition, the direction of maximum horizontal in-situ stress is NE-SW determined by induced fractures picked up by image logs. The NE orientation faults and fractures are increased and the effect of karst fluid on reservoirs is enhanced. However, the NW direction faults and fractures are restricted. Therefore, the fault-karst system is mainly observed along the NE main fault. The fluid alteration has two influences on the reservoir quality. On the one hand, the fluid alteration can dissolve the carbonate rocks and expand the reservoir scale; on the other hand, the fluid will carry mud and sand, which will reduce the reservoir space scale. The distance from the fault will affect the types of reservoir space of the fault-karst system. Away from the fault, the reservoir space will change from caves and fractures to facture-vug, and finally into small-scale dissolution vugs. Therefore, it should not only pay attention to seismic beads features but also to the size of available reservoir space on logging during drilling. This study can provide insight into the different types of fault-karst system models, and help clarify the main controlling factors of the fault-karst system scale.

In-situ stress prediction in ultra-deep carbonate reservoirs of Fuman Oilfield, Tarim Basin of China

The Fuman Oilfield in Tarim Basin has great potential for ultra-deep carbonate oil and gas resources, and is an important area for future storage and production increase. The present-day in-situ stress field is critical during the exploration and development. However, no systemic investigations have been carried out in this oilfield. Therefore, in this study, the present-day in-situ stress field in the Ordovician carbonate reservoir is predicted and analyzed based on well log calculation and geomechanical numerical modeling. The results indicate that, 1) NE-SW-trending is the dominant horizontal maximum principal stress (SHmax) orientation. The vertical principal stress is the maximum principal stress, showing the Ordovician reservoir is under a normal faulting stress regime. 2) The distribution of in-situ stresses in the Ordovician carbonate reservoir is heterogeneous, which is mainly controlled burial depth and fault/fracture development. High stress magnitudes in the Yingshan Formation are mainly in the southeastern part of Fudong area, Fuman Oilfield. The present-day horizontal differential stress mainly ranges from 27 MPa to 30 MPa in the Yingshan carbonate reservoir. iii) Natural fractures are generally stale under the present-day in-situ stress state. Fractures that parallel to the SHmax orientation with high fracture dip angle are easier to be reactivated. The results are expected to provide geomechanical references for further oil and gas development in the Fuman Oilfield of Tarim Basin.

Middle to Upper Ordovician stable carbon isotope stratigraphy and sedimentary facies in the Shunbei and Tahe areas, northern-central Tarim, China

The Shunbei area, located in northern-central Tarim (Northwest China), represents a new, important area for petroleum and gas exploration. This study presents high-resolution, conodont biostratigraphically controlled stable carbon isotope data from 540 samples of four deeply buried (∼7000–8000 m) wells in the Shunbei area and its nearby Tahe area spanning the Middle to Upper Ordovician for detailed chemostratigraphic correlations. Additionally, thin sections are obtained from the two studied areas for sedimentary facies analysis. The δ13C data from Shunbei area correlate well with the results from Tahe area and other outcrops in northwestern Tarim. The major global δ13C excursions of the mid-Darriwilian Isotopic Carbon Excursion (MDICE, ∼+1‰) and the Guttenberg Isotopic Carbon Excursion (GICE, ∼+1‰) are recognized from the upper Yijianfang and the Lianglitag formations respectively. Two minor (∼+0.5‰) δ13C positive peaks from the Yingshan Formation of the Shunbei area are comparable to lower Darriwilian strata of platform facies in Tarim, providing a potential tool for correlating lower Darriwilian hydrocarbon reservoirs, such as the Yingshan and Yijianfang formations in the Tarim Basin. A major sedimentary gap is detected in the lower Qiaerbak Formation of the Shunbei area and the upper Yijianfang Formation of the Tahe area, indicating an erosional event occurred in the early Sandbian which could be correlated with the globally tectonic-active interval during the Darriwilian–Sandbian transition. The completely preserved MDICE and GICE in the Shunbei area, as well as its relatively deeper-water sedimentary environment in comparison with the Tahe area, collectively suggest that the Shunbei area is of significance for further geochemical studies on the Middle and Late Ordovician environments and their resources.

Karst architecture characterization of deep carbonate reservoir using image logs in Tarim Basin, West China

This paper has conducted a high‐resolution characterization of a deeply buried carbonate karst in the Tarim Basin of China by utilizing the FMI (Full‐bore Formation Micro Image) method integrated with seismic data, drilling cores and thin sections. It was found that the type and scale of wellbore karst architecture are mainly controlled by karst paleogeomorphology and vertical faults. The karst highlands and slopes, respectively, develop vadose karst and phreatic karst, with different compositions of karst elements. In terms of the karst thickness in the wellbore, the vertical fault development zone is almost twice that of the non‐development zone, both for the vadose karst and phreatic karst. Furthermore, the vertical fault is also able to promote the development of mixing karst. The mixing zone is located 55 m below the water table, with a strata thickness of approximately 25 m, a net reservoir thickness of 5–15 m, and an average porosity of 4%–8%, which serves as an effective hydrocarbon reservoir in the Yingshan Formation. The results will provide insights into the vertical division of karst reservoir and help predict reservoirs using well logs.

Hypogenic origin of paleocaves in the Ordovician carbonates of the southern Tahe oilfield, Tarim basin, northwest China

The Lower-Middle Ordovician paleocaves at the depths of over 6000 m in the southern Tahe Oilfield have been important objectives of petroleum production in the Tarim Basin. 3D seismic data, drilling well data, image logs, and cores were integrated to delineate the development characteristics, spatial distribution, and morphology of paleocave reservoirs in the Tahe Oilfield. The petrographic, geochemical features and fluid inclusion measurements of the cave and fracture-filling calcite in the Tahe Oilfield were subsequently performed. The formation mechanism of paleocaves in the southern Tahe Oilfield was revealed through the comparative analysis of paleocaves in the north and south of the Tahe Oilfield. The paleocaves in the northern and southern Tahe Oilfield are extensively distributed 0–150 m and 160–350 m away from the regional unconformity, respectively. Different from the diverse paleocave morphologies including single cave passages, anastomotic mazes, and branchwork passages in the northern Tahe Oilfield, the paleocaves in the southern Tahe Oilfield are dominated by isolated chambers and rising shafts. Additionally, almost all the cave and fracture-filling calcite cements in the northern Tahe Oilfield coexist with gray-green sand and clay. While, the cave-filling calcite cements are relatively clean, without any terrigenous minerals. Furthermore, the calcite samples in the northern Tahe Oilfield show δ13C and δ18O values varying from −5.90‰ to −0.39‰, and from −13.47‰ to −7.23‰, respectively. The calcite cements in the southern Tahe Oilfield present similar δ13C values with the Ordovician limestone samples and relatively more negative δ18O values in a range between −15.02‰ and −12.29‰. Compared with the calcite samples in the norther Tahe Oilfield, the cave-filling calcite in the southern Tahe Oilfield shows a narrower range of 87Sr/86Sr ratios varying from 0.709342 to 0.709689. In particular, large amount of two-phase aqueous inclusions observed in the calcite from the southern Tahe Oilfield display relatively higher homogenization temperatures ranging from 90.8 °C to 205.9 °C. These significant differences on the paleocaves between the north and south of the Tahe Oilfield suggest that the paleocaves in the south are overprinted by both gravity-driven deep circulating meteoric water during the early Hercynian and the ascending thermal water possibly occurring from the second episode of the middle Caledonian to the early Hercynian. The paleocaves in the southern Tahe Oilfield are interpreted to be generated by hypogene karst under the constraints of strike-slip fault system and lithostratigraphic sequence in the Ordovician in confined settings. This study is an important supplement to the previous Ordovician meteoric karst model in unconfined zones of the Tahe Oilfield. It indicates that the stratabound isolated paleocaves developed in the lower submember of the Upper Yingshan Formation in the southern Tahe Oilfield have significant exploration potentials.

Microfacies and Reservoir Space Characteristics of Carbonate Reservoir of the Ordovician Yingshan Formation in Northern Slope of Tazhong Uplift, Tarim Basin

As an important stratum of exploration in the Tarim Basin, which is the largest petroleum basin in China, Yingshan Formation has great potential. The relationship between the fine division of microfacies and reservoir space characteristics has not been systematically studied which restricts further exploration. On the basis of the analysis of core, thin section, logging data, imaging logging data (FMI), and field outcrop, the microfacies types and reservoir space characteristics of the Ordovician Yingshan Formation are systemically studied. The results show that (1) nine microfacies (Mf1-Mf9) and four microfacies associations (MA1-MA4) were identified within the Ordovician Yingshan Formation. MA1-MA2 represent a medium- to high-energy sedimentary background, and MA3-MA4 represent a medium- to low-energy sedimentary background. (2) Four reservoir space types were observed in the Yingshan Formation, which are dominated by fracture-dissolution pore type, followed by small dissolution vug type, seldom fracture type, and large-scale karst-cavity type. (3) High-energy microfacies are the material basis of favorable reservoirs. The high-energy microfacies associations are prone to small dissolution vug and fracture-dissolution pore types of reservoirs; large cave reservoirs can sometimes be developed in the high part of the sequence and form favorable reservoirs in the Yingshan Formation. The low-energy microfacies associations mostly appear as original strata or interlayer, but after the superposition of structural cracks, buried karsts, or epigenetic karsts, low-energy microfacies associations may form relatively small-scale fractured and fracture-dissolution pore types of reservoirs in this area.

Factors controlling the development of carbonate reservoirs of Ordovician Yingshan formation in the Gucheng area, Tarim Basin

Deep carbonate rocks are important targets for oil and gas exploration in China. In recent years, significant oil/gas discoveries have been made in the deep carbonate sequences in the Tarim, Sichuan, Ordos, and Bohai Bay basins. Despite significant oil/gas discoveries, large-scale exploration has not been conducted in the Gucheng area in the Tarim Basin. To break the bottleneck restricting the petroleum exploration in the Gucheng area, this study analyzed the factors controlling the formation of carbonate reservoirs of the Yingshan Formation in the Gucheng area in detail based on the basic geological conditions of the study area and the data from cores, thin sections, well logging, testing, and 3-D seismic survey. The inner shallow-ramp in the Gucheng area acts as the main sedimentary facies zone for the development of high-quality reservoirs. The grainstones formed in the high-energy environment of the inner shallow-ramp laid the foundation for subsequent reservoir development in the study area. The dolomitized shoal grainstones in the inner shallow-ramp have well-developed intercrystalline pores and intercrystalline dissolved pores due to later dolomitization, thus serving as high-quality reservoirs. Strike-slip faults are crucial to reservoir reformation and determine whether high production can be achieved in oil and gas exploitation in the study area. Moreover, later reformation by hydrothermal solutions also plays a constructive role in reservoir formation.

Understanding the role of sequence stratigraphy and diagenesis on the temporal and spatial distribution of carbonate reservoir quality: A conceptual modeling approach

Karst caves formed by meteoric water leaching are important oil/gas storage spaces in carbonate reservoirs. However, carbonate reservoirs are transformed by atmospheric freshwater due to long-term cumulative effects that take place over millions of years, and laboratory physical simulations are therefore difficult to conduct. The mechanism by which meteoric water leaching transforms the carbonate sequence stratigraphy is therefore unclear, which has hindered the exploration and development of oil/gas resources. In this study, a large set of geological test data was utilized to develop a solute transport numerical simulation model based on multi-phase flow and multi-component reaction. This was then applied to construct a conceptual model of a grade III sequence stratigraphy for a reservoir in the Yingshan Formation in the central Tarim Basin, China. The multi-stage leaching process of meteoric water during the periodic exposure of the sequence stratigraphy was simulated to explore the effects of the diagenetic transformation of meteoric water on sequence stratigraphy and the associated physical responses. Results revealed that the composition of carbonate rocks affects the degree of reservoir development under the sequence boundary. Calcite dissolution plays a strong role in meteoric water leaching. Carbonate strata with a high calcite content forms secondary pores relatively easily and dolomite can be preserved. Under open geological conditions, the transformation of reservoirs by meteoric water in different periods has the characteristics of continuity, indirectness, superposition, and heterogeneity. In addition to the mineral dissolution caused by the unsaturated solution of the underlying strata, the transformation was also affected by the superposition and indirect influence of the migration and infiltration of the overlying strata fluid. Under heterogeneous conditions, meteoric water preferentially flowed along the dominant channels, which could improve reservoir performance locally, and may form oil and gas migration channels or large karst caves. This investigation of the control of meteoric water during reservoir development provides a theoretical basis for the genetic mechanism by which a high-quality reservoir can develop under the carbonate sequence boundary.

Evolution process and factors influencing the tight carbonate caprock: Ordovician Yingshan Formation from the northern slope of the Tazhong uplift, Tarim Basin, China

The tight limestone of the Yingshan Formation (Ordovician) is a local tight carbonate caprock in the Ordovician Tarim Basin. It vertically overlaps with the hydrocarbon reservoirs. The diagenesis and evolution processes affect the closure of this type of caprock, but it has been scarcely studied. This research utilized detailed laboratory experiments such as thin section study, cathodoluminescence (CL) examination, isotopic analysis (carbon, oxygen, and strontium isotopes), fluid inclusion, and the well-logging data to evaluate the evolution process and factors influencing the tight carbonate caprock of the Ordovician Yingshan Formation from the northern slope of the Tazhong uplift, Tarim Basin, China. The tight carbonate rocks of the Yingshan Formation in the study area are mainly composed of micrite and grainstone. The caprock thickness of the upper Ying 1 and the upper Ying 2 members of the Yingshan Formation are 15–44 m and 8–80 m, respectively, showing good continuity. However, the seal rocks in the other members of the Yingshan Formation are discontinuous. Low energy facies zone, vertical vadose zone, slow flow zone, and zone with weak tectonic destruction are the main areas of caprock distribution. Karstification in the epigenetic stage and cementation in the burial stage are the crucial factors affecting the evolution and distribution of carbonate seal rocks. Paleogeomorphology, phreatic surfaces, original sedimentary environment, and fault distribution influence the karstification, thus enhancing the macro heterogeneity of caprock. Cementation is controlled by buried fluids (formation water, hydrocarbons, hydrothermal fluid) and improves the micro sealing of the caprock. Multistage tectonic movements and changes in the fluid environment constantly change the sealing property of the caprock, and the reservoir and seal rock can transform each other in the process of evolution. These results suggest carbonate reservoirs evaluation can thus be enhanced by combining caprock evolution to analysis effectiveness. This research explains the mechanism of the spatial distribution of carbonate caprocks and provides the theoretical basis for carbonate reservoirs exploration.

Diagenesis Sequence and Hydrocarbon Accumulation Period of the Ordovician Reservoir in Well Tashen-6, Tahe Oilfield, Tarim Basin, NW China.

The main types of diagenesis, diagenetic minerals and their formation time sequence in the Ordovician ultradeep (>7000 m total depth) carbonate reservoir represented by the Yingshan and Penglaiba Formations (well Tashen-6, Tahe Oilfied, Tarim Basin), are determined by applying microscopic observations, microscopic fluorescence detection, and cathodic luminescence analysis in petrographic thin sections. The distinct periods of reservoir diagenesis and hydrocarbon-related events are determined by analyzing the development characteristics of hydrocarbon inclusions and their relationship with the host minerals. The charging periods of hydrocarbon inclusions are identified by constraining the homogenization temperatures of inclusions. The obtained results indicate that the Ordovician Yingshan and Penglaiba formations have experienced at least three periods of hydrocarbon charging and one period of structural transformation. Their relative time sequence relationship with diagenesis processes is as follows: The limestone dissolution of the Yingshan Formation developed initially, and the first period of hydrocarbon charging occurred (during the late Caledonian). The second period of hydrocarbon charging occurred due to the continuous modification influence of dissolution (late Hercynian–early Yanshanian). The limestones of the Penglaiba Formation were exposed to strong tectonism during the second period of hydrocarbon charging in the Yingshan Formation; thus, intralayer microfractures were formed. Additionally, the first period of hydrocarbon charging in the Penglaiba Formation occurred together with the dolomite reservoir (late Hercynian–early Yanshanian). During the subsequent period, dissolution occurred again due to the continuous increase in burial depth. The third period of hydrocarbon charging developed concurrently with the early fractures (late Himalayan). Finally, the unceasing deepening of the strata accompanied by tectonic activity led to the early intergranular dissolution pores to be cut by late microfractures, which caused the crude oil to convert into bitumen through secondary modifications.

High-Frequency Sea-Level Cycle Reconstruction and Vertical Distribution of Carbonate Ramp Shoal Facies Dolomite Reservoir in Gucheng Area, East Tarim Basin

During the sedimentary period of the Ordovician Yingshan Formation, the carbonate platform of the Gucheng area in the Tarim basin was characterized by a distally steepened ramp. Relative sea-level changes exerted a strong influence on the shoal facie dolomite reservoirs of the 3rd Member of the Ordovician Yingshan Formation (the Ying 3 member), sedimented in the context of a shallow water environment on the carbonate ramp. However, previous studies that lacked high-frequency sea-level changes in the Gucheng area prevent further dolomite reservoir characterization. The current work carries out systematic sampling based on the continuous core from the upper and middle parts of the Ying 3 member in two newly drilled exploration wells (GC17 and GC601) and a series of geochemistry analyses, such as C-O isotope, Sr isotope, and rare earth elements (REE), which helps to investigate the features of the shoal facies dolomite reservoir development against high-frequency sea-level changes. With the help of Fischer plots of these two wells, high-density δ13C data (sample interval is about 0.272 m) were merged to construct a comprehensive curve, contributing to characterizing the high-frequency sea-level changes of the upper and middle parts of the Ying 3 member in the Gucheng area and validating the relationship between the pore-vug vertical distribution and high-frequency sea-level changes. Results revealed that the porosity of dolomite reservoirs increased when the high-frequency sea-level fell and decreased when it rose. Furthermore, the karst surface can be found at the top of the upward-shallowing cycle during the high-frequency sea-level falling; the pore-vug reservoirs are concentrated below the karst exposure surface, and porous spaces are more developed closer to the top of the cycle. The high frequency sea-level curve built in this study can be used as a standard for further research of regional sea-levels in the Gucheng area, and this understanding is highly practical in the prediction of shoal facies carbonate reservoir in carbonate ramp.

CO2-Water-Rock Interactions in Carbonate Formations at the Tazhong Uplift, Tarim Basin, China

The effects of CO2-water-rock interactions on the injectivity and safety of CO2 geological storage have drawn wide attention. The geochemical reaction mechanisms in carbonate formations after CO2 injection are still controversial. To better understand the transformation of injected CO2 in carbonates and the involved geochemical reactions, we first conducted autoclave experiments reproducing the in-situ conditions of the Lianglitage Formation, Yingshan Formation, and Qiulitage Formation at the Tazhong Uplift in the Tarim Basin. We then established a batch model using TOUGHREACT-ECO2H, validated with the experimental results, to simulate the long-term CO2-water-rock interactions. It was found that the initial mineral compositions and water chemistry have important effects on the CO2-water-rock interactions in carbonate formations. The experiment results show that the dissolution of calcite and dolomite dominates in the early reaction period. However, we still observed some secondary minerals, such as ankerite, montmorillonite, calcite, and dolomite. The CO2-water-rock reactions can be more dramatic when the contents of calcite and dolomite in carbonates are closer. Moreover, the long-term simulation results show that calcite, magnesite, and hematite are the main formed secondary minerals, whereas dolomite is the major dissolved mineral. This study is helpful for a better understanding of the CO2 mineral trapping mechanism in carbonate formations.

Dolomite genesis and dolomitization mechanisms of the Ordovician lower Yingshan Formation, Gucheng area, Tarim Basin, China

The massive dolomite reservoirs of the Ordovician formations in the Tarim Basin have significant gas exploration potential, but their occurrence and distribution are not clearly understood. This study collected core samples covering regional dolomite reservoirs from the lower Yingshan Formation in the Gucheng area. A combination of petrology, fluid-inclusion assemblage (FIA) microthermometry, and isotope geochemistry data were used to define the dolomitization process and its impact on reservoirs. Petrological and geochemical analyses reveal that various dolomites have different dolomitization mechanisms. In (pene)contemporaneous (PC) stage, on the western side of the east Tarim Platform, laminated dolomicrite (LDm) of the third Member (3rd Mm) was formed by microbial and sabkha dolomitization, whereas dolomicrite (Dm) of the fourth Member (4th Mm) generated from sabkha dolomitization in the slightly elevated salinity setting in an evaporative sea environment. The residual structural dolomite (RSD) of the 3rd and 4th Mm was characterized by reflux fluids on the central platform margin. The shallow-middle burial (SMB) stage was dominated by reflux dolomitization, precipitated structureless dolomite (SD) primarily associated with RSD. Bimodal geochemical characteristics indicate that dolomitizing fluids gradually transitioned to pore water consisting of evaporated seawater reserved in the strata. In the middle-deep burial (MDB) stage, hydrothermal burial effects were superimposed and transformed the dolomites near the fractures. Leopard dolomite (LD) in the 4th Mm is limited to faults and has seawater and hydrothermal geochemical characteristics, controlled by the superimposition of pore water in SMB stage and hydrothermal fluids in MDB stage. Massive dolomite (MD) in 3rd Mm, which is not completely limited to the fracture system, was derived from multiple superimposed dolomitization events and transformed by terrestrial water. Finally, the effect of dolomitization on reservoirs was investigated. It is concluded that the sabkha and reflux dolomitization in PC stage was the primary dolomitization mechanism and was the basis for the pore formation.

The thermal history in sedimentary basins: A case study of the central Tarim Basin, Western China

• Heat flow of the Shunbei area decreased since the Ordovician with a Permian peak. • The middle-lower Ordovician reservoirs experienced three stages of heating process. • Hydrocarbon charging periods of No.1, 5, and 7 strike-slip fault zones are different. The reconstruction of thermal history is essential for the superdeep (>6,000 m) hydrocarbon exploration and development in the Central Tarim Basin, Western China. This study first recovered the burial histories of five typical wells in the Shunbei area (the Central Tarim Basin) based on the drilling data and calculated the erosional thicknesses using the well-tie profiles. The thermal histories and hydrocarbon charging periods of the Shunbei area were studied with the measured equivalent vitrinite reflectance ( R equ ) and fluid inclusion assemblage homogenization temperature data, respectively. Except for some abnormal values, most R equ data with 0.45%–1.48% gradually increase with depth. The thermal modeling revealed that the heat flow in the Shunbei area gradually decreased since the Early Ordovician, with an abrupt peak of ∼45–47 mW/m 2 during the Early Permian. The temperature histories of the Ordovician Yijianfang and Yingshan Formation reservoirs in the Shunbei area could be divided into three stages of rapid heating (1.27–1.36 °C/Ma), slow heating (0.07–0.09 °C/Ma) and rapid heating (1.20–1.28 °C/Ma). We recognized three hydrocarbon charging periods in the No. 1 strike-slip fault zone, which occurred at ∼435–420 Ma, ∼263–220 Ma, and 18–3 Ma, respectively, while the No. 5 and 7 strike-slip fault zones only experienced the first two periods. This study provides new insights into the thermal history and hydrocarbon accumulation in the Shunbei area, which would be very useful for further superdeep oil and gas exploration and resource evaluation in the Central Tarim Basin.