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According to the theory of sequence stratigraphy based on continental transgressive-regressive (T-R) cycles, a 500 m continuous core taken from the second member of Kongdian Formation (Kong 2 Member) of Paleogene in Well G108-8 in the Cangdong Sag, Bohai Bay Basin, was tested and analyzed to clarify the high-frequency cycles of deep-water fine-grained sedimentary rocks in lacustrine basins. A logging vectorgraph in red pattern was plotted, and then a sequence stratigraphic framework with five-order high-frequency cycles was formed for the fine-grained sedimentary rocks in the Kong 2 Member. The high-frequency cycles of fine-grained sedimentary rocks were characterized by using different methods and at different scales. It is found that the fifth-order T cycles record a high content of terrigenous clastic minerals, a low paleosalinity, a relatively humid paleoclimate and a high density of laminae, while the fifth-order R cycles display a high content of carbonate minerals, a high paleosalinity, a dry paleoclimate and a low density of laminae. The changes in high-frequency cycles controlled the abundance and type of organic matter. The T cycles exhibit relatively high TOC and abundant endogenous organic matters in water in addition to terrigenous organic matters, implying a high primary productivity of lake for the generation and enrichment of shale oil.

Abstract The spontaneous imbibition (SI) process in shale oil reservoirs is not only influenced by capillary force, but also by the osmotic pressure between the fracturing fluid and formation water in the nanopores media. In this study, experimental methods are used to investigate the mechanisms of osmosis in the SI, taking into account the presence of initial formation water in shale oil reservoirs. To investigate the effect of osmosis, SI experiments were performed on the fine-grained felsic shale of the Qikou sag of Dagang oilfield. Low-field NMR testers and high-precision electronic balances are utilized for the measuring of oil–water migration. The results show that, when S w ≠ 0, high-salinity fluid SI can be divided into four stages: initial imbibition stage, drainage stage, secondary imbibition stage and stationary stage; when S w = 0, there is no drainage stage of high-salinity fluid SI; when S w ≠ 0 or S w = 0, low-salinity fluid SI can be called the “osmosis-enhanced SI”; and we have found that “newly formed pores or microfractures” as well as reducing salinity can promote SI. This article presents a systematic study of SI of shale oil reservoirs under the influence of osmosis, which provide useful information for reservoir numerical simulation and development program design.

As the main factors affecting stable and high production and the production regularity of lacustrine shale oil are unclear, the theoretical understandings, key exploration and development technologies, development effect and production regularity of lacustrine shale oil have been analyzed and summarized based on 700 m cores taken systematically from Paleogene Kong 2 Member of 4 wells in Cangdong sag, over 100 000 analysis data and formation testing data. Three theoretical understandings on shale oil enrichment and high production have been reached: (1) High-quality shale with “three highs and one low” is the material base for shale oil enrichment. (2) Medium—slightly high thermal evolution degree is the favorable condition for shale oil enrichment. (3) Laminar felsic shale is the optimal shale layer for oil enrichment in semi-deep lake facies. Key exploration and development technologies such as shale oil enrichment layer and area evaluation and prediction, horizontal well pattern layout, shale oil reservoir fracturing, optimization of shale oil production regime have been established to support high and stable shale oil production. Under the guidance of these theoretical understandings and technologies, shale oil in Cangdong sag has achieved high and stable production, and 4 of them had the highest production of over 100 tons a day during formation testing. In particular, Well GY5-1-1L had a daily oil production of 208 m3. By April, 2022, the 28 wells combined have a stable oil production of 300–350 tons a day, and have produced 17.8×104 t of oil cumulatively. It is found that the shale oil production of horizontal well declines exponentially in natural flow stage, and declines in step pattern and then tends stable in the artificial lift stage. Proportion of light hydrocarbons in produced shale oil is in positive correlation with daily oil production and decreases regularly during production test.

A negative tectonic area refers to the relatively lower tectonic area from slope to deep depression in a basin. The theoretical understanding of oil and gas accumulation in the negative tectonic area is based on three accumulation models of "lithologic and stratigraphic reservoirs in the area of sag, lacustrine shale oil in deep depression, and coalbed methane in syncline area". Integrating with previous studies on reservoirs in negative tectonic area, a systematic knowledge system of oil and gas accumulation mechanisms and distribution laws in negative tectonic areas has been formed. The key points are as follows: (1) From the center to the margin of basin, oil and gas reservoirs are distributed in a circular order in the transverse direction, and enrich radially along dominant petroleum migration pathways in the longitudinal direction. (2) Due to different tectonic backgrounds, the sedimentary characteristics, hydrocarbon accumulation conditions, accumulation mechanism and distribution laws of negative tectonic area are greatly different from those of the positive tectonic area. (3) In deep depression and low slope, in situ hydrocarbon accumulation is dominant, and unconventional oil and gas distribution ring (inner ring) is formed. On medium and high slopes, passing oil and gas are captured, and lithologic and stratigraphic oil and gas distribution ring (central ring) is formed. While in positive tectonic area, oil and gas accumulate in high locations, and tectonic oil and gas distribution ring (outer ring) is developed. (4) An exploration idea based on the theory of oil and gas accumulation in negative tectonic area was proposed as follows. The target area should be integrately considered and exploration should be carried out from ring to ring. Migration pathways must be reguarded as a clue to seize oil and gas targets. The theory of oil and gas accumulation in negative tectonic area has effectively guided the discovery of a large number of subtle and unconventional oil and gas reservoirs in the slopes and deep sags of Jizhong and Huanghua depressions, and enriched the modern theory of oil and gas accumulation. It has an important practical guiding value for the exploration of deep complex subtle reservoirs and unconventional reservoirs in the negative tectonic area of basins.

The Paleogene Shahejie Formation (Shahejie Fm) of Bohai Bay Basin has ignited unprecedented interest among scholars owing to another breakthrough achieved in shale exploration besides Kongdian Fm. However, this breakthrough, particularly high output submember—the third Member of the Paleogene Shahejie Formation (Mbr 3) lacks elaborate study on paleoenvironment and its covariations with organic matter (OM) concentration for shales, which impedes the further acquaintances of OM distribution and shale oil–gas possibility. The present study adopts advanced approaches to gain insights into the deposition process of shales in the Mbr 3, Qikou Sag, Bohai Basin. Petrological characteristics, organic geochemistry, and an abundant element database offer a solid foundation for performing master controlling factor analysis and establishing deposition modes for OM accumulation and preservation. Our results indicate that the entire Mbr 3 has potential. Shales in the Mbr 3 of the Shahejie Fm were deposited under a temperate environment with weak weathering and sediment influx surrounded by suboxic freshwater. However, the organic-enriched shale in the upper Mbr 3 is a more desirable exploration layer. Disparate paleoproductivity support and clastic detritus sediment afforded a diminished consequence of OM accumulation between the upper Mbr 3 (L1–L2 in this study) and middle Mbr 3 (L3 in this study). Our investigation suggests that the OM concentration in the shales was dominated primarily via paleoproductivity and secondarily via redox reactions. Clastic sediment influx sets the primarily passive effect, while effect of salinity was minimal. Two sedimentary modes were established to interpret the variability between L1–L2 and L3 during the two depositional stages. The lake expansion period of L3 resulted in shales being deposited mainly on a gentle slope with a shallow water column containing insufficient OM susceptible to redox condition compared to the optimal state in the semideep lake in L1–L2 stage. The results provide an integrated reconstruction of paleosedimentary environment for the target layer and offer a solid basis for further exploration through coupling relationship between sedimentary environment and OM enrichment in the study area.

Based on core, thin section, X-ray diffraction, rock pyrolysis, CT scanning, nuclear magnetic resonance and oil testing data, the macro and micro components, sedimentary structure characteristics, of Paleogene Kong 2 Member in Cangdong sag of Huanghua depression and evaluation standard and method of shale oil reservoir were studied to sort out the best shale sections for shale oil horizontal wells. According to the dominant rock type, rhythmic structure and logging curve characteristics, four types of shale lithofacies were identified, namely, thin-layered dolomitic shale, lamellar mixed shale, lamellar felsic shale, and bedded dolomitic shale, and the Kong 21 sub-member was divided into four quasi-sequences, PS1 to PS4. The PS1 shale has a porosity higher than 6%, clay content of less than 20%, and S1 of less than 4 mg/g; the PS2 shale has well-developed laminar structure, larger pore and throat size, better connectivity of pores and throats, high contents of TOC and movable hydrocarbon, S1 of over 4mg/g, clay content of less than 20%, and porosity of more than 4%; PS3 shale has S1 value higher than 6 mg/g and clay content of 20% - 30%, and porosity of less than 4%; and PS4 shale has lower TOC content and low oil content. Shale oil reservoir classification criterion based on five parameters, free hydrocarbon content S1, shale rhythmic structure, clay content, TOC and porosity, was established. The evaluation method of shale oil sweet spot by using the weighted five parameters, and the evaluation index EI were proposed. Through comprehensive analysis, it is concluded that PS2 is best in quality and thus the dual geological and engineering sweet spot of shale oil, PS3 and PS1 come next, the former is more geologic sweet spot, the latter more engineering sweet spot, and PS4 is the poorest. Several vertical and horizontal wells drilled in the PS2 and PS3 sweet spots obtained high oil production. Among them, Well 1701H has produced stably for 623 days, with cumulative production of over 10000 tons, showing bright exploration prospects of Kong 2 Member shale oil.

Compared with marine facies shale strata, lacustrine shale strata are more complicated in geological conditions, and thus more difficult to explore and develop. To realize economic exploration and development of lacustrine shale oil, the geological regularities of accumulation and high yield of retained movable petroleum in shale should be understood first. In this work, taking the shale strata of Kong 2 Member and Sha 3 Member in the Paleogene of Huanghua depression in the Bohai Bay Basin as examples, based on the previous joint analysis results of over ten thousand core samples and the latest oil testing, production test and geochemical data of more than 30 horizontal wells, accumulation conditions and models of retained movable petroleum in lacustrine shale were studied comprehensively. The study shows that at moderate organic matter abundance (with TOC from 2% to 4%), shale strata have the best match between oil content and brittleness, and thus are rich in oil and good in fracability. Moderate ancient lake basin size and moderate sediment supply intensity are the internal factors leading to best coupling of organic matter abundance and brittle mineral content in the shale formation. Moderate thermal evolution maturity of Ro of 0.7%–1.0% (at burial depth of 3200 to 4300 m) is the interval where oil generation from thermal evolution and oil adsorption by kerogen in shale layers match best, and retained movable petroleum is high in proportion. Moderate diagenetic evolution stage (3200 to 4300 m in the middle diagenetic stage A) is conducive to the formation of a large number of dissolved pores and organic matter pores, which provide storage space for shale oil enrichment. Moderate development degree of natural fractures (without damaging the shale oil roof and floor sealing conditions) is conducive to the storage, seepage and preservation of shale oil. The research results have overthrown the general understanding that high organic matter abundance, high maturity, and high development degree of natural fractures are conducive to shale oil enrichment, and have guided the comprehensive evaluation of shale oil and gas sweet spots and well deployment in the second member of the Kongdian Formation in the Cangdong sag and the Shahejie Formation in the Qikou sag. Industrial development of the shale oil in Kong 2 Member of the Cangdong sag has made major breakthrough, and important signs of shale oil have been found in Sha 3 Member of the Qikou sag, demonstrating huge exploration potential of lacustrine shale oil.

High-yielding oil wells were recently found in the first member of Paleogene Shahejie Formation, the Binhai area of Qikou Sag, providing an example of medium- and deep-buried high-quality reservoirs in the central part of a faulted lacustrine basin. By using data of cores, cast thin sections, scanning electron microscope and physical property tests, the sedimentary facies, physical properties and main control factors of the high-quality reservoirs were analyzed. The reservoirs are identified as deposits of slump-type sub-lacustrine fans, which are marked by muddy fragments, slump deformation structure and Bouma sequences in sandstones. They present mostly medium porosity and low permeability, and slightly medium porosity and high permeability. They have primary intergranular pores, intergranular and intragranular dissolution pores in feldspar and detritus grains, and structural microcracks as storage space. The main factors controlling the high quality reservoirs are as follows: (1) Favorable sedimentary microfacies of main and proximal distributary gravity flow channels. The microfacies with coarse sediment were dominated by transportation and deposition of sandy debris flow, and the effect of deposition on reservoir properties decreases with the increase of depth. (2) Medium texture maturity. It is shown by medium-sorted sandstones that were formed by beach bar sediment collapsing and redepositing, and was good for the formation of the primary intergranular pores. (3) High content of intermediate-acid volcanic rock detritus. The reservoir sandstone has high content of detritus of various components, especially intermediate-acid volcanic rock detritus, which is good for the formation of dissolution pores. (4) Organic acid corrosion. It was attributed to hydrocarbon maturity during mesodiagenetic A substage. (5) Early-forming and long lasting overpressure. A large-scale overpressure compartment was caused by under-compaction and hydrocarbon generation pressurization related to thick deep-lacustrine mudstone, and is responsible for the preservation of abundant primary pores. (6) Regional transtensional tectonic action. It resulted in the structural microcracks.

The significance of lacustrine shale oil has gradually become prominent. Lacustrine shale has complex lithologies, and their reservoir properties are quite various. The multi-scale pore structure of shale controls the law of shale oil enrichment. Typical lacustrine shale developed in the Member 2 of Kongdian Formation in Cangdong sag, Bohai Bay Basin. The lithofacies and multi-scale storage space of this lacustrine shale have been systematically studied. 1. The mineral composition is quite different, and the lithofacies can be summarized into siliceous, carbonate and mixed types. The rock structure can be summarized into laminated, layered, and massive types. 2. The pores are diverse and multi-scale. Interparticle pores contribute the main storage space, especially the interparticle pores of quartz and dolomite. 3. The physical properties of the massive shales is relatively inferior to those of layered and laminatedtypes, and it presents the characteristics of " laminated >layered > massive ". The developed laminae can significantly improve the space and seepage capacity of lacustrine shale. 4. Clay minerals provide the main nano-scale storage space, but they are often filled in pores and reduces the shale brittleness, which have destruction effects.

The study of lithofacies identification and its distribution characterizations of shales is essential for the geological evaluation of shale oil exploration and development. In this study, core description, optical microscope, total organic carbon (TOC) content, whole-rock X-ray diffraction (XRD), Rock-Eval pyrolysis, X-ray fluorescence (XRF), scanning electron microscope (SEM), and well logging data were used in the qualitative analyses of the geochemical and mineralogical characteristics of shales. Since the work of detailed lithofacies identification is still lacking in the previous studies, based on TOC, sedimentary structure, and ternary diagram of siliceous minerals, carbonate minerals, and clay minerals, eight predominant lithofacies were identified in the shale of the second member of the Kongdian Formation (Ek2): ① low to moderate organic matter content (referred to as “low to moderate TOC”) massive calcareous mudstone (C-1); ② low to moderate TOC laminated calcareous mudstone (C-2); ③ low to moderate TOC layered mixed calcareous mudstone (C-3); ④ moderate to high organic matter content (referred to as “moderate to high TOC”) laminated calcareous siliceous mudstone (S-1); ⑤ moderate to high TOC massive mixed siliceous mudstone (S-2); ⑥ moderate to high TOC laminated clayey siliceous mudstone (S-3); ⑦ low to moderate TOC massive mixed mudstone (M-1); ⑧ moderate to high TOC layered calcareous/siliceous mixed mudstone (M-2). The favorability of lithofacies was investigated based on TOC, pyrolysis parameters, pore structures, and specific mineral contents. S-1 was currently considered as the most favorable lithofacies with excellent hydrocarbon potential, high amount of free hydrocarbon, and abundant organic pores; S-2, S-3, and M-2 were considered as favorable lithofacies, while C-1 and M-1 were ranked as the least-promising lithofacies in the research area. The lateral distribution of the shale is quite different, and Ek23 contains the most favorable lithofacies for shale oil exploration.

Under the general trend of stepping-up oil and gas exploration and development in China, the unconventional oil and gas resources such as shale oil and gas have become an important alternative. Abundant continental shale oil resources are developed in Member 2 of Kongdian Formation (Ek2) of the Cangdong sag in the Huanghua depression of the Bohai Bay Basin which has complex structure, strong heterogeneity, and large buried depth, thus, the geological characteristic of shale oil accumulation and key exploration technologies are investigated on base of the system coring of 635.8 m, centimeter-level fine description and analysis of over ten thousand samples, therefore, the “four accurate” technologies have been developed, including the accurate selection of sweet spot areas, accurate drilling of sweet spot layers, accurate fracturing of sweet spot layers and accurate measure of fracture-rich sections. The results show that the continental shale strata in Member 2 of Kongdian Formation can be divided into three types: felsic shale, limy dolomitic shale and mixed shale. The Member 2 of Kongdian Formation is characterized by diverse mineral components, abundant laminae, good source rocks and medium thermal evolution degree, tight reservoir with rich micropore and microfractures, high oil saturation and brittle mineral content, indicating good prospect of shale oil exploration. Through sedimentological study, TOC and Ro evaluation, brittle mineral calculation, and seismic fusion inversion, a total of 126 km2 of Class I sweet spots in Member 2 of Kongdian Formation was delineated comprehensively, which guides horizontal well deployment (Well GD1701H and Well GD1702H). The drilling rate of sweet spot of these two horizontal wells is up to 96% by tracing thin layer through fine calibration, locating compartment through fine inversion, ensuring window entry through precise positioning, and tracking sweet spot through adjustment. The ''one-excellent and five-highs” criterion is used to select perforation points to realize differential designs of fracturing intervals and clusters. Micro-seismic and potentiometry monitoring show that the artificial fractures formed by volumetric fracturing are 300–400 m long and 120 m high, and control a volume of about 0.07 km3. The maximum daily oil production of two horizontal wells is 48 t and 66 t respectively to realize the economic exploitation of shale oil reserves controlled by fractures. The shale oil exploration in Member 2 of Kongdian Formation shows that the continental shale oil has great potential of exploration and development, and the above technologies are critical and effective for shale oil efficient recovery. Keywords: Shale oil, Sweet spot evaluation, Fracturing intervals and perforation clusters, Member 2 of Kongdian formation, Cangdong sag, Bohai Bay Basin

Based on the merged 3D seismic data, well logging, formation testing, analysis and testing data, the structural evolution, sedimentary reservoirs, thermal evolution of source rocks were investigated of Paleogene Kongdian Formation in the trough area of Cangdong sag, Bohai Bay Basin. A conventional-unconventional hydrocarbon accumulation pattern in the trough area of rifted basin was revealed. The reservoir forming elements in the trough area of Cangdong sag have a zonation feature in terms of reservoirs and source rocks. There are two types of reservoir forming models, primary trough and reformed trough. The formation and evolution of trough controlled the orderly distribution of conventional oil to unconventional oil in the trough. Particularly, structural reservoirs occur in the upper part of the trough, stratigraphic-lithologic reservoirs are likely to form in the delta front deposits at the outer ring of trough, the middle ring transitional belt is the favorable site for tight oil reservoirs, while the fine grain deposits zone in the inner ring is shale oil and gas exploration area. The study has pointed out the new domains and directions for searching reserves in the secondary exploration of mature oilfields. Key words: rifted basin, trough hydrocarbon accumulation, stratigraphic-lithologic reservoir, shale oil, orderly accumulation, Bohai Bay Basin, Cangdong sag, Paleogene Kongdian Formation

Abstract: A deep understanding of the basic geologic characteristics of the fine-grained shale layers in the Paleogene 1st sub-member of Kong 2 Member (Ek21) in Cangdong sag, Bohai Bay Basin, is achieved through observation of 140 m continuous cores and systematic analysis of over 1 000 core samples from two wells. Basic geological conditions for shale oil accumulation are proposed based on the unconventional geological theory of oil and gas. The shale rock system mainly developed interbedded formation of felsic shale, calcareous and dolomitic shale and carbonates; high quality hydrocarbon source rock formed in the stable and closed environment is the material base for shale oil enrichment; intergranular pores in analcite, intercrystalline pores in dolomite and interlayer micro-fractures make tight carbonate, calcareous and dolomitic shale and felsic shale effective reservoirs, with brittle mineral content of more than 70%; high abundance laminated shale rock in the lower section of Ek21 is rich in shale oil, with a total thickness of 70 m, burial depth between 2 800 to 4 200 m, an average oil saturation of 50%, a sweet spot area of 260 km2 and predicted resources of over 5×108 t. Therefore, this area is a key replacement domain for oil exploration in the Kongdian Formation of the Cangdong sag. At present, the KN9 vertical well has a daily oil production of 29.6 t after fracturing with a 2 mm choke. A breakthrough of continental shale oil exploration in a lacustrine basin is expected to be achieved by volume fracturing in horizontal wells. Key words: shale oil, fine grained deposits, horizontal well, volume fracturing, shale reservoir sweet spot, Paleogene Kongdian Formation, Cangdong sag, Bohai Bay Basin

Based on systematic coring of 500 m of Kong 2 Member of the Paleogene Kongdian Formation in Cangdong sag of Bohai Bay Basin, identification and XRD (X-Ray Diffraction) analysis of over 1000 thin sections, a simplified method to quantitatively calculate contents of fine grained minerals with conventional logging data such as acoustic travel time (AC) and density log (DEN) has been proposed, and a quick lithologic identification “green mode” has been worked out in this study. By fitting the relationship between normalization of logging curves and mineral content measured by XRD, the mineral contents of sections or wells not cored can be calculated to identify lithology. With this method, several dolomite sweet spot intervals and one sandstone sweet spot interval have been found in the Kong 2 Member of Cangdong sag, where high production oil and gas flows have been tapped from drilled wells. The study shows that the dolomite is in band distribution and enriched in local parts of the study area. This method is applicable to lithologic identification of fine grained deposits in front delta-lake basin center, especially lithologic identification of mud and dolomite dominated fine grained deposits with low sand content of semi-deep, deep lake facies. Key words: fine grained deposits, lithologic identification, dolomite, tight oil, reservoir sweet spot, Paleogene Kongdian Formation, Cangdong sag, Bohai Bay Basin

Based on seismic, logging, formation testing, core and lab test data, this study analyzed the sequence division, facies features of deep water deposits and modes, development of large-scale gravity flow, reservoir physical properties and their main controlling factors, and proposed a classification standard and prediction method of favorable exploration areas in deep water area of the Bin1 oil layers of the lower sub-member of the first member of Paleogene Shahejie Formation in Banqiao-Qibei slope zone of Qikou sag, Bohai Bay Basin. The Bin1 oil layers can be divided into three fifth-order sequences, each less than 100 m thick; a set of gravity flow deposits were formed under deep water background in the slope zone, which contains sedimentary micro-facies such as main channel, distributary channel, channel margin, inter-channel mudstone, and turbidite sand sheet in areas without channels, and, in space, has inherited and constructive development features of multistages. It is a sedimentary sequence of fan delta – distal subaqueous fan – deep lake, and every distal subaqueous fan formed by gravity flow can be divided into inner-, middle- and outer fans. The cross-facies transported sands which are sourced from higher-sand-content major sands of delta front can form high quality reservoirs with an average porosity of 15.1% and geometric average permeability of 5.1×10−3 μm2. The main channel and distributary channel of distal subaqueous fan are the most favorable exploration zones. Key words: deep water deposit, slope zone, sandy debris flow, subaqueous fan, lithologic reservoir, Shahejie Formation, fault lake basin, Qikou sag, Bohai Bay Basin

Abstract: Based on analysis of successive and whole cores over six hundred meters from the second Member of Kongdian Formation (Kong 2 Member) of fine-grained facies zone in central lake basin of Cangdong sag, ten thousands of systematic and joint analysis data, and the matching logging data, the sedimentary characteristics in the fine-grained facies zone is examined by means of traditional petroleum geology, sedimentology, and new theory and method of tight oil and gas generation. The Kong 2 Member is the sedimentation during the maximum lake flooding in the Kongdian Formation. Fine-grained sedimentation of half-deep lake to deep lake are widely distributed, the fine-grained feldspar-quartz contained sedimentary rock, fine-grained mixed sedimentary rock, and dolomite are developed, and the clay rock is poorly developed. The fine-grained rocks have these features as follows: more rock compositions, less advantageous minerals; more debris minerals, less clay minerals; more analcites, less pyrites; more brittle minerals, less quartz; more rock types, less oil shale; more tight reservoirs with less non-cracked reservoirs; more high-quality source rocks, less non-source rocks; more high-frequency cycle sediments, less single sediments. These understandings updated the former view that only mud shale source rock was dominantly developed in the closed continental lake. With the help of geologic research and matched engineering technologies, several exploration wells have produced industrial oil flows, showing a good prospect of tight oil exploration in fine-grained facies zones of continental lacustrine basins. Key words: fine-grained sediment, fine-grained facies, tight oil, geologic features, exploration prospect, Cangdong sag, Bohai Bay Basin