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The Longmaxi and Niutitang Formations are typical shale reservoirs in southeastern Chongqing. Since the Paleozoic, southeastern Chongqing has experienced multistage tectonic movement and diagenesis, resulting in the formation of a large number of natural fractures. In shale reservoirs, fractures not only provide seepage channels for oil and gas migration but also play an important role in the oil and gas reservoir space. Natural fractures provide a flow path from source rock to reservoir during oil filling and connect hydraulic fractures, matrix pores, and boreholes during production. Therefore, identifying the main factors that control the development of natural fractures in the Longmaxi and Niutitang Formations in southeastern Chongqing has a guiding significance for the efficient development of shale reservoirs in this area. By considering the Longmaxi and Niutitang Formation shales as case studies, using field outcrops, drilling cores, and other data, and conducting X-ray diffraction (XRD) analysis as well as total organic carbon (TOC) measurements, the quantitative parameter characteristics and main factors that control the development of natural fractures in reservoir shales were examined in detail. The results obtained showed that there are three types of fractures in the lower Paleozoic shale in southeastern Chongqing, which are structural, diagenetic, and abnormal high-pressure fractures. Among them, the fractures in the Longmaxi Formation shale, which are relatively abundant, predominantly consist of low-angle and high-angle inclined fractures, while the Niutitang Formation predominantly consists of high-angle fractures. Additionally, the investigation of fracture size and fracture density, as well as correlation analyses, showed that the fractures of the Lower Paleozoic shale are predominantly micro fractures that play a key role in improving reservoir seepage. It was also noted that the development of fractures is affected by several factors, including tectonic stress, mineral composition, organic carbon content, and rock thickness. The degree of fracture development was found to be positively correlated with brittle mineral and quartz contents, and high organic matter contents also significantly favored the formation micro fractures.

The reservoir quality of tight sandstones is generally determined by the depositional environment, burial depth and diagenesis. However, the relationship between reservoir quality of tight sandstones and the controlling factors are very complicated. Deeply buried Middle-Upper Jurassic Shishugou Group sandstones are important tight oil sandstone reservoirs in the central Junggar Basin of NW china. The individual sandstones are thin (2–20 m, av.7.8 m), overlain by an unconformity and directly connected to the source rock by a well-developed fault system. The diagenetic system and reservoir quality of the Shishugou Group sandstones in the Central Junggar Basin was investigated using petrology, mineralogy, pore characteristics, sedimentary facies, burial and thermal history, knowledge of hydrocarbon charging, fault/fracture system, diagenetic reactions and sequence, controlling factors of reservoir quality, paleo-porosity reconstruction and formation mechanism of high-quality reservoir. The thin-section observation, scanning electron microscope (SEM and BSD), granularity analysis, Xray diffraction (XRD) and fluorescence microscope, 3D seismic and well logging analysis were used in this research. The top unconformity and faults system were the pathways for water and hydrocarbon migration. Organic acids and meteoric water with high concentrations of CO2 were the main fluids that caused dissolution of volcanic rock fragments, feldspar, analcime cement and early calcite cement. The paleo-porosity variation of Shishugou Group sandstones was analyzed quantitatively by the inversion and back stripping with the constrain of diagenetic evolution history and theoretical compaction models. The comprehensive researches including the origin of pore waters, origin of acidic fluids, fault/fracture system and hydrocarbon charging history, show that near-surface or meteoric diagenetic reactions of Shishugou Group sandstones closely beneath top unconformities were in open (geochemical) diagenetic systems, and that diagenetic (geochemical) systems were relatively open, especially in the faults-developed zone during burial process. The central sandstone body of the point bar, underwater distributary channel and distributary channel, which were cut through by the oil source fault, retained high-quality reservoir. The reservoir quality can be mostly predicted as a function of sedimentary environment (sedimentary facies and provenance), diagenetic reactions and burial history with the constrain of source rocks, diagenetic system and fault system.

The physical property heterogeneity of tight sandstones was mainly caused by complex alteration of various diagenesis combinations during burial process. However, diagenetic evolution of different diagenesis combinations which generally result in the strong difference and heterogeneity of physical property and pore structure is rarely well understood. The Middle Permian lower Shihezi Formation is one of the most important tight gas sandstone reservoirs in the Hangjinqi area of Ordos Basin, China. The reservoir heterogeneity of lower Shihezi Formation, which was caused by the differential diagenesis combination, is crucial to efficient exploration and development. Evolution mechanism of differential diagenesis combination and its effect on the reservoir quality in the tight lower Shihezi Formation sandstone in the Hangjinqi area of Ordos Basin was investigated by means of thin-section description, cathodoluminescence (CL) imaging, X-ray diffraction (XRD), scanning electron microscopy (SEM), and homogenization temperature of fluid inclusions. The lower Shihezi Formation sandstones can be divided into four diagenesis combination types according to the reservoir characteristics and diagenetic relationship. The main diagenetic sequence was mechanical compaction-chlorite rim-early pore-filling calcite cementation-dissolution-authigenic kaolinite-quartz cementation-late calcite cementation. Differential diagenesis combination was mainly controlled by the petrological characteristics, microfacies, and fault. Low content of rock fragment and high content of detrital quartz were beneficial to the compaction resistance and cementation. The moderate content of pore-filling calcite was conducive to pore space protection and feldspar dissolution. The faults control dissolution and differential diagenesis combination by influencing the migration of acid fluids. Moderate compaction-moderate cementation-moderate dissolution type (BBB type) and weak compaction-moderate cementation-strong dissolution type (CBA type) were in favour of high-quality reservoir development.

The diagenetic evolution is an important controlling factor of shale gas reservoir. In this study, based on field outcrop and drilling core data, analytical techniques including X-ray diffraction (XRD), field emission scanning electron microscope combined with a focused ion beam (FIB-FESEM), and energy dispersive spectroscopy (EDS) analysis were performed to determine diagenetic evolution of the Longmaxi Formation shale , and reveal effect of diagenetic evolution on the shale gas exploration and development in the Sichuan Basin, Southwest China. The diagenetic evolution can be divided into eodiagenesis, mesodiagenesis, and teleodiagenesis. The eodiagenesis and mesodiagenesis were subdivided into two and three evolution stages respectively both in Basin margin and center. Absorbed capacity and artificial fracturing effect of the Longmaxi Formation shale gas was related to mineral composition, which was influenced by the sediment characteristics and diagenetic evolution. Diagenetic system in the Basin margin was more open than that in Basin center due to different structural burial history. The more open diagenetic system consisting of more micro-fractures and soluble constitute (e.g. feldspar) was in favour for formation and preservation of secondary dissolved pores and organic pores in the Basin margin. The relatively closed diagenetic system with stronger compaction resulted in deformation of pore space in the central Basin due to different burial history and diagenetic evolution.

Microfractures are key for migrating and aggregating hydrocarbon source rocks and fracturing oil-gas exploitation in tight reservoirs. In this study, rock samples from the Lucaogou Formation tight reservoirs in Xinjiang, China, were studied using multidisciplinary techniques to investigate the genetic types and main control factors of microfractures. Results indicated that the Lucaogou Formation mainly developed diagenetic microfractures followed by tectonic microfractures, with slight formations of granular microfractures. These observations were used to clarify the relationship between the development of microfractures and the pore fluid content, lithology, mineral composition, and stratum thickness. A higher pore fluid content corresponded to a lower compressive strength of the rocks and a larger ring count, resulting in a higher probability of failure and microfracture formation. Tight reservoirs containing more quartz and carbonate minerals were found to develop more microfractures. Quartz grains showed fractures at the margins under stress, which increased the pore permeability of rocks. Carbonate minerals tended to form microfractures owing to corrosion. Microfracture formation mechanisms differed depending on lithology, and microfractures were found to develop most in dolomite and dolomitic siltstones and least in mudstone. Muddy rocks developed fewer tectonic fractures because they can easily absorb stress and undergo plastic deformation. Within a certain stratum thickness range, the average single-well fracture space and stratum thickness showed positive correlations. Moreover, the fracture space increased and the fracture density decreased as the stratum thickness increased. When the stratum thickness was less than 2.5 m, the fracture space increased linearly with the stratum thickness, and when the stratum thickness was greater than 2.5 m, the fracture space remained constant. This study will provide an essential scientific basis for enhancing tight oil recovery.

The dolostone reservoir of the Middle Permian Maokou Formation in Eastern Sichuan has good prospects for oil and gas exploration. Study of dolomitizing genesis of the Maokou Formation is essential for predicting the distribution of the dolostone reservoir. Petrography, in situ geochemistry, Sr-Mg isotopes, and fluid inclusions were carried out on samples from the Maokou Formation in Eastern Sichuan in order to discuss the dolomitizing process. Based on mineral and textural characteristics, dolomites were divided into four components: partially clouded dolomite (PCD), mosaic-like dolomite (MLD), cloudy-centered and clear-rimmed dolomite (CACD), and saddle dolomite (SDD). Results indicate that the Maokou Formation in Eastern Sichuan mainly experienced two stages of dolomitization. PCD, MLD, and cloudy-centered dolomite (CCD) were formed during the early dolomitization. They all show turbid crystal planes and bright orange-red CL and have similar trace element contents, 87Sr/86Sr ratios, and rare-earth patterns, indicating that they might be formed in the same fluid. This is a period when dolomitizing fluids mainly migrated along pores or microcracks and replaced protogenetic calcites, which occurred in the shallow burial stage of the Maokou Formation before the Late Permian. Clear-rimmed dolomite (CRD) and SDD were formed in the late stage of dolomitization. They all have clean crystal planes and darkly red CL. CRD of the ERY profile has trace element contents, 87Sr/86Sr ratios, and rare-earth patterns similar to SDD of the HLCH profile and Well TL6, inferring that both may be formed in the same fluid. Combined with high SrO contents and homogenous temperatures of fluid inclusions of CRD and SDD and Mg-isotopic compositions, they were generated by hydrothermal dolomitization. The hydrothermal fluid stage is related to the movement of the Emeishan Large Igneous Province, which was made up of basaltic magmatic fluids mixing with the surface water. The hydrothermal fluid mainly migrated upwards along structural fractures or faults and filled in structural fractures, occurring in the Late Permian to Middle-Late Triassic.

The shallow-marine turbidite fans in the Upper Miocene Huangliu Formation of the Yinggehai Basin in the northwestern South China Sea (SCS) provide an excellent opportunity to understand their sedimentary processes in a shelf depositional environment. The down-slope gravity flow processes and along-slope bottom-current reworking processes of shallow-marine turbidite fans were interpreted by using seismic, well logging, core, petrographic, geochemical, and petrophysical data. Several depositional elements were identified in the shallow-marine turbidite fans, namely, channel-fill high-density turbidites (HDTs), channel-fill low-density turbidites (LDTs) and associated frontal splays, sand-rich/mud-rich lobe deposits, and bottom-current reworked channel-fill/lobe deposits. Deep U-shaped (or V-shaped) seismic reflections and low root-mean-square (RMS) amplitudes characterize the channel-fill HDTs that consist of massive fine-grained sandstones with mud clasts. The channel-fill LDTs, characterized by V-shaped or worm-shaped reflections, mostly consist of normally graded, laminated and rippled, very fine-grained sandstones. Frontal splays are generally associated with channel-fill LDTs. The sand-rich lobe deposits show continuous high-amplitude sheet-like reflections and consist of HDTs and LDTs, whereas the mud-rich lobe deposits show continuous moderate-amplitude reflections and consist of muddy debrites. The bottom-current reworked sandstones (BCRSs), which comprise well-sorted, fine-grained sandstones with traction-current structures, are usually located in the upper parts of thick sandbodies. The variability of depositional elements from large-scale channel-fill HDTs with strong basal erosion in fan-1 to small-scale channel-fill LDTs in fan-2 is closely linked with sea-level fluctuations that result in variable gravity-flow energy and sediment input. However, the reoccurrence of large-scale channel-fill HDTs in fan-3 at sea-level highstands may possibly be attributed to enhanced sediment input from the source areas. Down-slope flow transformation from turbidity flows into muddy debris flows within an individual channel-lobe complex (CLC) resulted in a dramatic increase in clay content and resultant decreasing reservoir quality from the channel-fill HDTs to the mud-rich lobe deposits, because muddy sediments are incorporated into the precursor turbidity flows and turbulence is suppressed. Additionally, it is suggested that the widely developed traction-current structures and tidal signatures (double mud layers, mud-draped ripples, discrete wavy bedding, internal truncation surface, and convex-up laminae) are the products of reworking by internal waves and -tides. During periods of sea-level highstands, the upper parts of gravity-flow sandstones would undergo bottom-current reworking, thus resulting in the retransportation of muddy fines and the formation of reworked sandstones with traction-current structures and tidal signatures. In this study, a combination of traction-current structures, tidal signatures, vertical sequences showing sharp upper contacts and non-gradational upper contacts, and trace elements is considered to be convincing diagnostic criteria in distinguishing reworked sandstones from gravity-flow sandstones. The representative bottom-current reworked sandstones should be preferable hydrocarbon targets in further exploration because of their better reservoir properties compared with gravity-flow sandstones. This research offers some insight into gravity-flow processes and bottom-current reworking processes in a shallow marine environment.

The lithofacies characteristics of fine sedimentary rocks affect the hydrocarbon generation capacity and reservoir performance of tight oil and are closely related to the sedimentary environment. On the basis of field outcrops and core observations, combined with logging data, thin section identification, grain size analysis, organic carbon analysis and physical property tests, the fine sedimentary facies of the Chang 7 Member in the Ordos Basin are studied. Nine lithofacies types are identified in the study area, including planar and trough cross-bedding sandstone facies (Spt), massive bedding sandstone facies (Sm), slump deformation fine sandstone facies (Ssd), sand-mud interbedded facies (Sb), rippled siltstones facies (Fr), horizontal bedding mudstone facies (Mh), massive bedding mudstone facies (Mm), horizontal bedding shale facies (S) and tuff facies (T). Lithofacies types Spt, Ssd, Fr are developed in the gentle slope belt in the northeastern study area, which is a delta front-semi-deep lake sedimentary environment and mainly contains underwater distributary channel and estuary dam sedimentary microfacies types. Lithofacies types such as Sm, Sb, Mh and T are developed in the southwestern steep slope zone of the study area, which is a channel-type gravity flow sedimentary environment, and sedimentary microfacies types such as near-source channels, far-source channels and leading-edge dolomites are developed. In the vertical direction, during the Chang 73 sedimentation period, the lake surface area and water depth reached their maxima, and S and Mh lithofacies developed, which were conducive to the formation of high-quality hydrocarbon source rocks for tight oil. During the Chang 72-Chang 71 sedimentation period, the lake basin shrank, and the reducibility of the water body weakened. The delta front sand body composed of Spt lithofacies in the northeast and the sandy clastic flow sand body composed of Sm in the southwest constitute the main tight oil reservoirs. High-quality hydrocarbon-generating lithofacies and favourable reservoir lithofacies are spatially superposed and developed, forming two source-reservoir combinations in the Chang 7 tight oil source-reservoir formation and near source-reservoir formation.

China shale oil, which is preserved in lacustrine shale with strong heterogeneity and relatively low maturity, has been a research hotspot of unconventional resources. However, controlling factors of shale oil enrichment and resource potential evaluation restricted efficient exploration and development of lacustrine shale oil. On the basis of well logging data, TOC content, Rock-Eval pyrolysis values, thermal maturity, 100 oil saturation data, and pressure coefficient, the core observation, X-ray diffraction analysis, physical property analysis, scanning electron microscopy, CT scan, well logging interpretation, and volumetric genesis method depending on three-dimensional geological modeling were used to determine enrichment factors and evaluate the resource potential of Qingshankou Formation shale oil in the Southern Songliao Basin. Shale oil was mainly enriched in the semideep and deep lake shale of K2qn1, with the high capacity of hydrocarbon generation and favorable petrological and mineralogical characteristics, pore space characteristics, and physical properties in the low structural part of the Southern Songliao Basin. The three-dimensional geological resource model of Qingshankou Formation lacustrine shale oil was determined by the key parameters (Ro, TOC, and ) of shale oil in the favorable zone of the Southern Songliao Basin, northeast China. The geological resource of shale oil, which was calculated by two grid computing methods ( and ), was, respectively, and . The great shale oil resource indicates a promising future in the exploration and development of Qingshankou Formation shale oil of the Southern Songliao Basin.

Two oolites of the Lower Triassic Feixianguan Formation on the margins of the Kaijiang-Liangping (K-L) Trough in the NE Sichuan Basin were deposited in the upper parts of similar shallowing-upward sequences, and are capped by a subaerial exposure surface. However, they have contrasting diagenetic histories and reservoir qualities: the sour oolitic dolostone reservoirs on the eastern side of the K-L Trough have significantly higher porosities than the sweet oolitic limestone reservoirs on the western side. A petrologic and geochemical comparison of the two oolites was carried out so as to investigate the role of diagenetic processes on the co-evolution of oolite structure and pore network. On both sides of the K-L Trough, eogenetic meteoric digenesis led to an inversion of texture with ooid dissolution and interparticle cementation in the oolite layers close to the exposure surface. The layers relatively far from the exposure surface mostly underwent recrystallization in association with connate seawater, resulting in neomorphic fabrics in the ooid cortices and less interparticle calcite cement. During the subsequent dolomitization, the early-formed oolite structure were mostly inherited by the dolostones on the eastern side, with fabric-retentive textures in the layers close to the exposure surface and fabric-destructive textures in the layers far from the exposure surface. During deeper burial the oolitic limestones were subjected to extensive calcite cementation and porosity loss, whereas the dolostones generally retained more porosity and have better connectivity of the micropore network. The higher porosity of the dolostones is ascribed to lesser chemical compaction and associated cement precipitation, rather than the creation of new pores. Porosity enhancement is closely dependent on the absence of the calcite cement related to pressure solution. The heterogeneity of the dolostone fabrics and the porosity was further enhanced by the subsequent recrystallization and the dissolution-reprecipitation process induced by thermochemical sulfate reduction, during which small amounts of net porosity are calculated to have been created. The oolitic limestones are the best reservoirs close to the exposure surface, whereas the most porous dolostones with the highest degree of fabric destruction are commonly some distance from the exposure surface. These results provide some insights into the diagenetic controls on oolite structure and porosity, and have implications for predicting the porosity of deeply-buried oolitic reservoirs, so have global theoretical and practical significance.

Previous studies have reported faunal changeover and carbon isotope excursions through the Wuchiapingian–Changhsingian (W C) transition, a geologically significant period of time that bridged two major extinction events: the end-Permian and Guadalupian–Lopingian boundary mass extinctions. However, the causes of the carbon isotope excursion across the W C interval remain poorly understood. This study presents sedimentological and geochemical data from three well-exposed sections in the lower Yangtze region, South China. A strong negative carbon isotope excursion in organic matter is recorded across the W C interval and correlates well with other sections in South China. Sedimentology and elemental geochemistry indicate that the lower Yangtze region experienced a distinct regional deepening during the W C transition inducing bottom water redox changes increased primary productivity. Possible causes of the negative carbon isotope excursion at the W C boundary include selective preservation of organic matter during early diagenesis under anoxic/euxinic conditions and changes in terrigenous organic matter input. Both of these factors were controlled by regional relative sea-level rise triggered by contemporaneous tectonic movements.

Order acceptance and scheduling problem is considered in a flow shop where the objective is to decide on the orders to accept and schedule the accepted orders so as to maximize total net revenue. An effective shuffled frog-leaping algorithm (SFLA) is proposed, in which population is divided into memeplexes using tournament selection and the search process within each memeplex is done on the best solution of the memeplex. We assess the performance of SFLA using an extensive set of instances. The experimental results show that SFLA can provide the promising results on the considered problems.