Yanchang Formation Research Articles

Oil and gas accumulation periods and charging path of continental lake basin: A case study of the Chang 9-Chang 10 oil reservoir in the Yanchang formation in the Ordos Basin

The Chang 9-Chang 10 Formation in the Ordos Basin has been explored intensively in recent years. However, the accumulation timing and migration paths of the oil in the formation reservoir are still unclear. This study uses reservoir geochemistry and software basin modeling to study the hydrocarbon accumulation periods of the oil-bearing sandstones in four wells in different areas of the basin. The results show that the principal oil charging period of the Chang 9-Chang10 formation was from 100 Ma – 130 Ma. The Chang 10 oil reservoirs in the Shanbei area and the Chang 9 oil reservoirs in the Jiyuan and Longdong areas experienced a single oil charging period. However, the Chang 9 oil reservoirs in the central lake area experienced two oil charging periods, from 190 Ma – 210 Ma and from 100 Ma – 130 Ma. The relative abundances of 1,8-/2,7-Dimethylcarbazole and PSNs/Ens carbazole indicate that there are hydrocarbon charging points in the source rocks of the Chang 7 formation in the Jiyuan area, and charging points of Chang 9 source rocks in the Shanbei area. Chang 9 crude oil in the Longdong area shows the characteristics of long distance migration. This study improves understanding of the hydrocarbon accumulation process in the Chang 9-Chang 10 oil reservoirs, which will inform and support future oil and gas exploration in the area.

Geochemical Characteristics of the Chang 7 Source Rocks of the Triassic Yanchang Formation in Ordos Basin, China: Implications for Organic Matter Accumulation and Shale Oil Potential

The Chang 7 member of the Upper Triassic Yanchang Formation in the Ordos Basin is considered to hold the main source rocks for conventional and unconventional oil and gas. The lamination or lithology alteration in vertical and lateral directions, even over a short distance, is a common feature in lacustrine source rocks. The differences in the geochemical characteristics of black shales, dark mudstones and interbedded sandstones have been scarcely reported, and their influences on the petroleum generation potential and shale oil potential are not clear. To this end, 22 core samples were collected from the Lower and Middle Chang 7 (C7-3 and C7-2) members of the Triassic from well CYX in the Qingcheng area. By conducting a series of geochemical analyses including TOC, Rock-Eval pyrolysis yields, bitumen extraction and quantification, and the separation and quantification of saturates, aromatics, resins and asphaltenes, along with biomarker analyses, several results were found. Firstly, the C7-3 and C7-2 source rocks are thermally mature and have entered into the stage of hydrocarbon generation. The C7-3 and C7-2 source rocks have a good to very good hydrocarbon generation potential especially the C7-3 black shales. Secondly, terrigenous source input is more abundant in C7-2, whereas the source input of phytoplankton, algae or microbial lipids is more abundant in C7-3. Moreover, a high TOC content basically corresponds to low wax indexes, terrigenous/aquatic ratios (TARs), and Pr/nC17 and Ph/nC18 ratios and high C27/C29 regular sterane ratios, which suggests that the source input of phytoplankton, algae or microbial lipids is favorable for OM accumulation. Third, analyses of the molecular composition of saturated fractions in shales and interbedded sandstones and the production index demonstrate the migration of petroleum from organic-rich source rocks to their organic-lean counterparts. The C7-2 dark mudstones could be considered as a potential “sweet spot” since their oil saturation index (OSI) was the highest among all the studied samples and they are more enriched in light aliphatic fractions.

Astronomical calibration of a ten‐million‐year Triassic lacustrine record in the Ordos Basin, North China

AbstractThe division of geological time is of general interest as this allows a better understanding of the geological processes and Earth's evolutionary history. Astrochronology provides a way to evaluate high‐resolution timescales of Earth's climate changes. Astronomical tuning of orbitally forced stratigraphic records to construct astronomical‐calibrated timescales has led to a progressive refinement of the geological timescale over the last decades. However, the Triassic remains a relatively weak link. A large lake with an area of 370 000 square kilometres formed in the Ordos Basin in North China during the Triassic Period. It has been demonstrated that the sedimentary deposits of the seventh Oil Member (Chang 7 Member) of the Triassic Yanchang Formation in the southern Ordos Basin responded to astronomically forced climate changes. However, the constructed astronomical‐calibrated timescales failed to reach an agreement. This study presents natural gamma ray logs and geochemical climate record of Fe and Al concentrations from the southern Ordos Basin margin that are dated precisely from 239 to 229 Ma by combined U–Pb dating and astrochronology. Therefore, the Chang 7 Member was deposited during the Ladinian and Carnian (late Middle through to earliest Late Triassic), this knowledge will refine the Triassic astronomical scale. The distribution of detrital zircon dates shows that the Chang 7 Member in the southern Ordos Basin has multiple provenance characteristics. The southern Ordos Basin is far from this provenance area, which is reflected in the low sediment accumulation rate (0.5 to 2.0 cm/kyr). In contrast to the lower part of Yanchang Formation underlying the Chang 7 Member, the large tuffs and seismites from the bottom of Chang 7 Member indicate the initiation of tectonic collision between North Qinling orogenic belt and the southern margin of the Ordos Basin.

Genetic Mechanisms and Identification of Low-Resistivity Pay Zones: A Case Study of Pengyang Area, Ordos Basin, China

The Triassic Yanchang Formation in the Pengyang area of Ordos Tianhuan depression is an important oil and gas formation. However, most of the oil pays in the study formations are low-resistivity or low-contrast reservoirs with low permeability, bringing challenges to the reservoir identification and evaluation by well logs. In this paper, we first measured the nuclear magnetic resonance (NMR), phase permeability, cation exchange capacity (CEC), and X-ray diffraction for core samples. Then, the genetic types for the low-resistivity pays were analyzed based on the experiment results, water analysis, and well log data collected. It was found that large variations of formation water salinity, high irreducible water saturation, and clay conductivity are the primary genetic types. Further, the random forest (RF) algorithm with sensitive parameter inputs was used to identify the oil, oil and water, and water layers. The anomaly of spontaneous potential (∆SP) that characterizes water salinity, the relative value of gamma ray log (∆GR) that describes the bound water content, resistivity, density, and acoustic logs were taken as sensitive logs according to the genetic analysis. Finally, this identification method was verified by comparison with the traditional crossplot method and oil test results. The identification accuracy of the RF is 90%, far higher than that by the crossplot method.

Oil generated overpressure of shale and its effect on tight sandstone oil enrichment: A case study of Yanchang Formation in the Ordos Basin, China

Oil generated overpressure (OGOP) is a key factor in tight sandstone oil enrichment. However, the relationship between the OGOP and oil enrichment remains unclear. On the basis of a physical simulation experiment and an oil injection experiment, an improved OGOP model based on the organic matter enrichment zone (OMEZ) was established, the attenuation of OGOP in shale was discussed, and the oil charging distance under different reservoir conditions was evaluated. The results showed that the OGOP starts from the OMEZ in the shale, is controlled by the OMEZ size, and decays during the expansion of the hydrocarbon expulsion microfractures. The attenuated OGOP acts as the initial migration force to push crude oil into tight sandstone reservoirs. The OGOP magnitude, OMEZ size and reservoir properties together determine the enrichment scale of tight sandstone oil. Driven by OGOP, the oil charging distance for the tight reservoir in the Yanchang Formation is only 66.7 m but 383.5 m for the conventional reservoir. These findings open a window to study the impact of shale internal structure on OGOP. The OGOP mathematical model and calculation method of various parameters in the process of oil accumulation proposed in this study are helpful for further understanding the mechanism of hydrocarbon generation, discharge, and enrichment.

Comprehensive Characterization Integrating Static and Dynamic Data for Dynamic Fractures in Ultra-Low Permeability Reservoirs: A Case Study of the Chang 6 Reservoir of the Triassic Yanchang Formation in the Ordos Basin, China

The generation of dynamic fractures during the process of water injection in ultra-low permeability reservoirs aggravates the heterogeneity of the reservoir, resulting in a rapid rise of water cut and directional flooding of the producers, which affects waterflood sweep efficiency and recovery. A dynamic fracture, as the geological feature of ultra-low permeability reservoirs, has a complex genetic mechanism and is difficult to characterize. Taking the Chang 6 reservoir of the Triassic Yanchang Formation in the Ordos Basin of central China as an example, this paper presents the characterization method and workflow of dynamic fractures. On the one hand, through the analysis of triaxial-compression rock-mechanic experiments and the mineral composition of the core, we evaluated rock brittleness in order to identify the lithology that can easily generate new fractures. On the other hand, beginning with the ancient tectonic stress field and combining the fracture characteristics of core and geological outcrop, the multi-fractal method and the probabilistic neural network were applied to identify the natural fractures and to quantitatively predict the intensity of natural fractures. Based on the rock brittleness evaluation and the natural fracture feature, the intensity of dynamic fractures was characterized by integrating the analysis of the bottom hole pressure, fracture pressure, and production response characteristics. A dynamic fracture is a “double-edged sword” during the waterflooding development of ultra-low permeability reservoirs. The premature activation and generation of dynamic fractures could lead to a worse development status. Nevertheless, the rational control and utilization of dynamic fractures play a positive role in improving oil recovery. Dynamic fractures are of great significance to the optimization and adjustment of well patterns for ultra-low permeability reservoirs. This can provide a reference for similar reservoirs.

New species of permochoristids (Insecta, Mecoptera) from the late Middle Triassic Tongchuan entomofauna in Shaanxi Province, northwestern China

ABSTRACT Permochoristidae is one of the most diverse mecopteran families in the fossil record, but only a few species have been previously described from China. Herein, we describe and illustrate two new permochoristid species, Mesochorista tongchuanensis sp. nov. and Agetopanorpa sinica sp. nov., from the late Middle Triassic of Yanchang Formation in Shaanxi Province. Mesochorista conjunctiva comb. n. is proposed for Qingochorista conjunctiva. Mesochorista tongchuanensis sp. nov. and Agetopanorpa sinica sp. nov. represent the sparse record of Mesochorista and Agetopanorpa in China, respectively. The new discovery adds to our knowledge of their distribution and palaeodiversity during the Triassic. urn:lsid:zoobank.org:pub:DA7ACCFF-D76E-492D-BB1B-37AC1CA5D264

Extremely High Primary Productivity and Organic Matter Enrichment in Lacustrine Sediments: Late Triassic Chang 73 Sub-Member, Western Ordos Basin

The 7th member of the Late Triassic Yanchang Formation is the most important Mesozoic source rocks and shale oil reservoir in the Ordos Basin. The hydrocarbon potential and organic matter enrichment mechanism of this organic-rich member are questioned because of the high heterogeneity of continental sediments. In particular, the lack of drilling in the Chang 73 sub-member of the western Ordos Basin (central Tianhuan Depression) hinders this understanding and the progress of shale oil exploration in this region. In this study, 87 samples were collected from the well DP1 in the Tianhuan Depression, western Ordos Basin, and analyzed for sedimentology and geochemistry to unravel the paleoenvironment, paleo-productivity, and hydrocarbon potential of the Chang 73 sub-member. The results show that the Chang 73 shale is characterized by ultra-high organic matter abundance (up to 10.60%), type II kerogen, and good hydrocarbon generation potential. An intermediate chemical weathering intensity suggests that this set of organic-rich shale was deposited in a warm and humid paleoclimate setting. Extreme organic matter enrichment in study area may be attributed to ultra-high productivity (hypereutrophic lake), superimposed terrigenous organic matter input, and transgression, rather than volcanism and anoxic bottom water. This model is different from those of Chang 73 interval with higher organic matter content in the central and southern parts of the Ordos Basin. Therefore, this study proposes that the Chang 73 shale in the central Tianhuan Depression is not a good target layer for shale oil exploration and that the favorable area may be located in an undisturbed deep lake with higher organic matter abundance (e.g., southwestern Ordos Basin).

Geochemical Characteristics of Crude Oil in Member 7 of Yanchang Formation in Yanchang Oilfield

Oil is an important primary energy for national production and life and national defense, and China is the key to rapid development. It is of great strategic significance to ensure the safe and efficient production of the oil industry. Yanchang Oilfield is located in the main position of oil and gas accumulation in Ordos Basin, China, and the seventh member of the Yanchang Formation is the area with the richest crude oil output at present. Studying the geochemical characteristics of its crude oil exploitation is of great engineering significance for the exploration of oil and gas content in similar terrain and the continuous improvement of oil extraction and refining technology. On the other hand, it also provides first-hand data and important technical references for the exploration and exploitation of oil and gas resources in other areas of the Ordos Basin. Firstly, this paper points out the important strategic significance of the petroleum industry for China’s development, and briefly describes the basic situation of Yanchang Oilfield and the basic characteristics of crude oil. The power and diversion system of the Yanchang Formation in Yanchang Oilfield are studied, and the main factors of formation and flow of deep reservoirs in the Yanchang Formation are explored, as well as the regional geological survey of the Yanchang Formation No. 7 in Yanchang Oilfield. The characteristics of crude oil and its maturity evaluation system are emphatically discussed, and the changes in crude oil characteristics under the action of microorganisms are comparatively studied. In this paper, Yanchang Oilfield, the main reservoir area of the Ordos Basin, is taken as the research object, and the formation mechanism of the oil reservoir and the geochemical characteristics of crude oil in the 7th member of the Yanchang Formation are deeply explored, so as to provide a preliminary theoretical study for the research of crude oil exploitation in reservoirs and other oilfields in the Ordos Basin.

Authigenic silica in continental lacustrine shale and its hydrocarbon significance

Taking lake basin shales of the Triassic Yanchang Formation in the Ordos Basin, NW China and the Cretaceous Qingshankou Formation in the Songliao Basin, NE China as research objects, the characteristics and origins of different types of silica in the shales have been studied by means of core observation, thin section identification, cathodoluminescence, X-ray diffraction analysis, scanning electron microscope (SEM), electron probe and rock pyrolysis. The results shows that the origins of silica include felsic mineral dissolution, tuffite devitrification, clay mineral transformation and siliceous mineral metasomatism. The silica formed by feldspar dissolution commonly appears as spots and veins, with low degree of crystallization, and is largely aqueous opal mineral, with an average SiO2 content of 67.2%. Silica formed by devitrification of tuffite mainly occurs in two forms, amorphous silica and authigenic quartz with better crystal shape. The authigenic silica formed during the transformation of clay minerals is embedded in the clay minerals in the form of micron-scale plates and small flakes, or mixed with clay minerals in a dispersed state. The authigenic quartz formed by siliceous mineral metasomatism is in better angular crystal shape, and has an average SiO2 content of 87%. The authigenic siliceous mineral content is positively correlated with the content of terrigenous felsic minerals. The pressure solution of felsic minerals is the main source of authigenic siliceous minerals, followed by the transformation of clay minerals, and the organic matter has some boost on the formation of authigenic silica. The authigenic siliceous materials of different origins have different geological characteristics and occurrence states from terrigenous quartz, which would affect the storage performance, seepage capacity and fracturing effect of continental shale. Although the organic-rich shale has high silica content, different from terrigenous quartz, authigenic silica in this kind of shale mostly floats and disperse in clay minerals, which would have negative effect on the formation of complex fractures in fracturing, fracture support ability after fracturing, and formation of effective seepage channels. Calculating the brittleness index of shale intervals only based on the composition of brittle minerals cannot accurately characterize mechanical characteristics of continental shale oil reservoirs, and would affect comprehensive evaluation and selection of continental shale oil “sweet spots”.

SOURCE ROCK POTENTIAL OF THE UPPER TRIASSIC CHANG 7 MEMBER IN THE WESTERN ORDOS BASIN, CHINA

The Chang 7 member of the Upper Triassic Yanchang Formation is an important source rock in the Ordos Basin, NW China. Previous studies of the unit have in general focused on the relatively deep‐water (∼25 m) anoxic, OM‐rich lacustrine mudstones which form the main Mesozoic source rock at fields in the south of the basin. However, this paper presents an integrated geochemical investigation of the relatively shallow‐water Chang 7 facies in a study area around Jiyuan field in the western part of the basin in order to evaluate its source rock potential. The results show that the Chang 7 source rock in the study area has a high content of Type II OM, and is interpreted to have been deposited in a suboxic‐anoxic lacustrine setting with a mixed input of aquatic and terrigenous organic matter. The results of 1D modelling of a well in the study area showed that the Chang 7 member entered the oil generation window from the Middle Jurassic. Oil‐oil correlations based on hierarchical cluster analysis and correspondence analysis showed that crude oils generated by deep‐water Chang 7 source rocks from wells in the south of the basin can be distinguished from Soxhlet‐extracted petroleum from reservoir sandstones in the study area. The compositional differences are inferred to be due to variations in source rock facies compositions. An oil – source rock correlation study showed that the shallower‐water Chang 7 source rock in the western part of the basin generated the hydrocarbons in core extracts of reservoir sandstones from wells in this area. The shallow‐water Chang 7 facies may therefore constitute an effective oil‐prone source rock in the western Ordos Basin.

Influence of Reservoir Pore-Throat Structure Heterogeneity on Water-Flooding Seepage: A Case Study of Yanchang Formation in Ordos Basin

The microscopic pore-throat structure of low-porosity and ultralow permeability sandstone reservoirs controls the seepage characteristics, which directly affects the water injection development efficiency of oilfields. Different from typical tight sandstone reservoirs, macropores and mesopores are more developed in the pore-throat structure of this type of reservoir, which changes the dominance of micropores over seepage capacity. Based on the full-range pore-throat structure characterization method and fractal theory, many experimental methods are used to study the influence of the microscopic pore-throat structure over the seepage characteristics in the Chang 9 reservoir in the Yanchang Formation of the Ordos Basin. The results of 12 typical samples show that the pore-throat structure has multifractal characteristics, and the occurrence degree of movable fluid and seepage capacity vary greatly, showing strong microscopic heterogeneity. Following characterization of the full-range pore-throat structure, the relative proportion of macropores and mesopores determines the physical properties of the reservoir. The pore-throat scale and structural heterogeneity have a significant impact on porosity, while the pore-throat structure connectivity has a crucial impact on permeability. Quartz provides resistance to compaction and preserves more primary pores. Additionally, the relationship between clay minerals and physical properties is not significant. Only illite and I/S mixed layers have a slight effect on permeability reduction. Furthermore, laumontite cementation is the key factor in the destruction of the pore-throat structure. Porosity has a significant effect on movable fluid occurrence and is more closely related to the two-phase seepage. Permeability controls the oil displacement efficiency in the anhydrous period, and porosity controls the oil displacement efficiency in the final period. The fractal dimension has some significant controls on the pore-throat structure, which are reflected in the fact that the higher the homogeneity of macropores is and the higher the heterogeneity of mesopores and micropores is, the better the reservoir development will be. In particular, the degree of macropore development guarantees reservoir quality. The control of the fractal dimension on the seepage capacity is complex, especially for mesopores and micropores; the higher the degree of heterogeneity is, the stronger the seepage capacity will be. The occurrence of movable fluid is significantly affected by the scale and heterogeneity of the pore-throat structure, which is reflected as stronger heterogeneity of the pore-throat structure and poorer relative seepage capacity.

Characteristics of Sedimentary Microfacies of Late Triassic Yanchang Formation in North-Central Ordos Basin

Sedimentary microfacies refer to the smallest unit with unique rock structure, structure, thickness, rhythm, and other sedimentary characteristics and certain plane configuration rules in the subfacies zone. Sedimentary microfacies research is the most basic sedimentary unit formed in the same or basically the same sedimentary background. Sedimentary microfacies marks are the key to the division of sedimentary microfacies. If we cannot find effective marks, we cannot correctly divide sedimentary microfacies. These marks are mainly obtained from detailed observation of cores. On the premise of fully understanding the regional tectonic background of the study area, the sedimentary facies and sedimentary subfacies types within the oilfield control range are determined. In this study, the single-well facies and profile facies are deeply analyzed by observing and describing the cores of Chang 2 reservoir group in Huangjialing, north-central Ordos Basin, and comprehensively utilizing the data of coring, logging, and core analysis and testing. The basic sedimentary characteristics, types of sedimentary facies and microfacies, and plane distribution of sedimentary facies in this area are systematically studied. The results show that there are mainly three microfacies: underwater distributary channel, natural dike, and distributary bay. Among them, the microfacies of the underwater natural dike are relatively undeveloped, mainly the microfacies of underwater distributary channel and underwater distributary bay. These studies have important practical significance for the later exploration of oil and gas in Ordos Basin.

Study on Enhancing Shale Oil Recovery by CO2 Pre-Pad Energized Fracturing in A83 Block, Ordos Basin

The Ordos Basin is rich in shale oil resources. The main targeted layers of blocks A83 and X233 are the Chang 7 member of the Yanchang Formation. Due to extremely low permeability, a fracturing technique was required to enhance oil recovery. However, after adopting the stimulated reservoir volume-fracturing technology, the post-fracturing production of the A83 block is significantly lower than that of the X233 block. For this problem, the dominating factors of productivity of the two blocks were analyzed using the Pearson correlation coefficient (PCC) and the Spearman rank correlation coefficient (SRCC), showing that the main reason for the lower production of the A83 block is its insufficient formation energy. To solve this problem, the CO2 pre-pad energized fracturing method was proposed. To study the feasibility of CO2 pre-pad energized fracturing in the A83 block, an integrated reservoir numerical simulation model of well A83-1 was established based on the idea of integration of geology and engineering. Additionally, the productions within five years after conventional volume fracturing and CO2 pre-pad energized fracturing were compared. The results show that compared with conventional volume fracturing, the cumulative oil production of CO2 pre-pad energized fracturing increases by 11.8%, and the water cut decreases by 16.5%. The research results can guide the subsequent reservoir reconstruction operation in the A83 block and provide new ideas for fracturing in the future.

Hydrocarbon accumulation model influenced by “three elements (source-storage-preservation)” in lacustrine shale reservoir-A case study of Chang 7 shale in Yan’an area, Ordos Basin

The organic-rich shales of the Chang 7 Member in the Yan’an Formation of the Yan’an area, Ordos Basin is a hot spot for lacustrine shale gas exploration. In this paper, taking the Chang 7 Member shale in the Yan’an area as an example, the main controlling factors of lacustrine shale gas accumulation and the prediction of “sweet spots” are systematically carried out. The results show that the Yanchang Formation shale has the complete gas generating conditions. Shale gas accumulation requires three necessary accumulation elements, namely gas source, reservoir and good preservation conditions. The dynamic hydrocarbon generation process of the Chang 7 shale reservoirs is established according to the thermal simulation experiments of hydrocarbon generation, and the mechanism of catalytic degradation and gas generation in the Chang 7 Member under the background of low thermal evolution degree is revealed. The enriched authigenic pyrite can catalyze the hydrocarbon generation of organic matter with low activation energy, thereby increasing the hydrocarbon generation rates in the low-mature-mature stage. Different types of pores at different scales (2–100 nm) form a multi-scale complex pore network. Free gas and dissolved gas are enriched in laminar micro-scale pores, and adsorbed gas is enriched in nano-scale pores of thick shales, and silty laminates can improve the physical properties of the reservoir. This is because the laminar structure has better hydrocarbon generation conditions and is favorable for the migration of oil and gas molecules. The thickness of the lacustrine shale in the Chang 7 Member is between 40 and 120 m, which has exceeded the effective hydrocarbon expulsion thickness limit (8–12 m). At the end of the Early Cretaceous, the excess pressure of the Chang 7 shale was above 3 MPa. At present, horizontal wells with a daily gas production of more than 50,000 cubic meters are distributed in areas with high excess pressures during the maximum burial depth.

Characterization of ultra-low permeability tight sandstone reservoir properties and criteria for hydrocarbon accumulation in Chang 6 member, Huaqing area, Ordos basin

How to accurately predict hydrocarbon enrichment and high-yield areas in ultra-low permeability sandstone reservoirs is a hot spot in petroleum geology. Taking the Chang 6 Member of the Yanchang Formation in the Huaqing Area, Ordos Basin as an example, the quantitative characterization and classification criteria of ultra-low permeability sandstone reservoir properties are systematically studied. The research results show that the seismic reflection characteristics of the Chang 6 Member show multi-stage wedge progradation characteristics, reflecting the sedimentary characteristics of the end of the delta front. The maturity of the minerals in the Chang 6 Member sandstones in the study area is very low, it indicates a low degree of transformation of sediments. Three types of sand bodies, namely sandy debris flow, turbidite, and slump rock sand bodies, were identified in the Chang 6 Member of the Yanchang Formation. The pore types of the Chang 6 Member in the study area include intergranular pores, dissolution pores (feldspar, detrital dissolution pores), and intercrystalline pores. Among them, the intergranular pores and feldspar dissolution pores are the most important storage spaces. The study also found that the test oil production is highly correlated with the cumulative thickness of the massive sandstone segments. Therefore, massive sandstone (sand debris flow) is the main contributor to oil well production, and its distribution directly controls the accumulation of oil reservoirs and the high production of oil wells. In addition, geological parameters affecting reservoir enrichment evaluation were analyzed, which include permeability, oil layer thickness, resistivity, and reservoir penetration rate. Finally, the evaluation criteria for hydrocarbon enrichment in the Chang 6 Member were formulated by the analysis of dominant facies, lithologic assemblages and electrical properties.

Adhesion Forces of Shale Oil Droplet on Mica Surface with Different Roughness: An Experimental Investigation Using Atomic Force Microscopy

In order to investigate the effect of rock surface roughness on the occurrence state of shale oil, muscovite mica was firstly characterized by performing atomic force microscopy (AFM). Two-dimensional (2D) images and the three-dimensional (3D) structure of the mica surface were obtained. Wettability of the micas was measured according to the sessile drop method using shale oil, collected from a lacustrine shale oil well drilling through the Yanchang Formation, Ordos Basin. Then, the adhesion forces between shale oil and mica surface with a different roughness were finely measured using AFM mounted with the shale oil modified probe tips. The adhesion force curves at the approaching and retract modes were obtained. The results show that the average roughness value of the mica samples was about 1 nm, while the maximum height was up to 4 nm. The contact angle between shale oil and mica ranged from 128.73° to 145.81°, and increased with increasing surface roughness, which can be described by the Wenzel model. The adhesion force between shale oil and mica also increased with an increasing contact area. Shale oil can fill the deep valleys on the rough surface of rocks and then form microscopic storage for oil droplets. The maximum adhesion force, reached at a distance of about 5–10 nm between shale oil droplets and micas, was between 14 and 30 nN. The adhesion force disappeared when the distance was larger than 40 nm. These indicate that shale oil in pores with a diameter of less than 10 nm was tightly adsorbed, and formed a layered accumulation pattern. Additional energy is needed to decrease the disjoining pressure and then separate shale oil from these tight pores. Shale oil is freely movable at pores with pore diameters of larger than 40 nm. This work provides a new insight about the interaction between shale oil and rock, and helps to understand the occurrence mechanism of shale oil.

Petrophysical characteristics of shale oil reservoirs in the Yanchang Formation, Ordos Basin

Petrophysical characteristics of shale oil reservoirs in the Yanchang Formation, Ordos Basin

Study on the Genetic Differences in Reservoir Characteristics Based on the Bidirectional Provenance of the Yanchang Formation in the Jiyuan Area, Ordos Basin, China

Ordos Basin is a Mesozoic sedimentary basin that underwent long-term evolution on the North China Craton. Many scholars have confirmed that in the Late Triassic, the basin was surrounded by ancient continents, and there were multiple provenance supply directions. Combined with the nature of the basement of the basin and the characteristics of the present structure, it is believed that the Jiyuan area is located in the central and western parts of the basin, spanning two first-level structural units, the Tianhuan Depression and the Yishan Ramp. This special geographical location makes Jiyuan area affected by bidirectional provenance. Controlled by the northwest and northeast depositional systems in the basin, Jiyuan area has accepted complex sedimentation and diagenesis, forming a low-porosity ultralow-permeability reservoir. However, the understanding of bidirectional provenance has been neglected in many previous studies on reservoir characteristics in the Jiyuan area. Therefore, the differential evolution of sedimentation and diagenesis caused by bidirectional provenance will cause serious deviations in the original understanding of reservoir characteristics in the Jiyuan area, which will inevitably affect subsequent exploration and development research work. In this paper, the mineral composition, physical properties, diagenesis, and diagenetic evolution of the Jiyuan area are studied by combining a large number of tests such as core physical properties, casting thin sections, scanning electron microscopy, cathodoluminescence, and X-ray diffraction. Then, the origins of reservoir development in two areas dominated by bidirectional provenance are analyzed and compared. Furthermore, the diagenetic facies are characterized by a cluster analysis of logging data, and finally, the reasons for the differences in reservoir distribution and the genetic mechanism between the Yinshan provenance area (YPA) and Alxa provenance area (APA) are obtained. The results show that, first, due to the different provenance, compared with the YPA, the reservoir pore space in the APA is better developed and the physical properties are better. Second, the clay mineral content and diagenesis are more important causes of reservoir differentiation, and the reservoir pores in the YPA are more affected by kaolinite and chlorite filling than those in the APA. Although more dissolution improvements have been obtained, the damage to the reservoir caused by cementation in the middle and late stages is extremely fatal, while the chlorite film in the APA reservoir has a better protection effect on the primary intergranular pores. Third, after the evolution of pores in the APA reservoir, more intergranular pores are preserved, and the distribution range of high-quality diagenetic facies is wider than that in the YPA. Finally, sedimentation is the basis for high-quality reservoir development, and good mineral content composition and favorable diagenetic transformation cause reservoir dissimilarity.

Fracture stratigraphy and mechanical stratigraphy in sandstone: A multiscale quantitative analysis

Mechanical stratigraphy can provide valuable knowledge for evaluating and predicting the distribution of structural fractures and in situ stress. The fracture stratigraphy and mechanical stratigraphy of the Triassic Yanchang Formation sandstone reservoirs in the Huaqing area of the Ordos Basin were investigated. In this study, borehole images were used to identify and characterize the orientation and properties of natural fractures, and array acoustic logs were used to analyze the rock mechanical properties. By considering the positional and scale cycles, the mechanical stratigraphy and areal fracture density at multiple scales were calculated, and the statistical correlation between them was analyzed. The results show that when the dynamic Young's modulus is less than 45 GPa, the areal fracture density generally increases with increasing Young's modulus; when the dynamic Young's modulus is greater than 48.5 GPa, the areal fracture density decreases with increasing dynamic Young's modulus. Additionally, the areal fracture density peaks when the dynamic Young's modulus is in the range of 45–48.5 GPa. In general, the dynamic Poisson's ratio is negatively correlated with the areal fracture density at multiple scales; when the dynamic Poisson's ratio is less than 0.24, the areal fracture density rapidly decreases as the dynamic Poisson's ratio increases; when the dynamic Poisson's ratio is greater than 0.24, the dynamic Poisson's ratio is only weakly correlated with the areal fracture density. The fitting results between the mechanical stratigraphy and areal fracture density show that the mathematical model relating these two parameters is related to the statistical scale.