The characteristics of laminae are critical to lacustrine shale strata. They are the keys to the quality of source rocks and reservoirs, as well as engineering operations in shale plays. This study uses organic geochemistry, thin section identification, X-ray diffraction, field emission scanning electron microscopy, and other analytical methods, to reveal the detailed lamination texture and vertical distribution of laminae in the second Member of the Kongdian Formation in Cangdong Sag. The principal results are as follows: (1) A classification of laminae is proposed to characterize reservoir and geochemical properties. The five types of laminae are as follows: feldspar-quartz laminae (FQL), clay laminae (CLL), carbonate laminae (CAL), organic matter laminae (OML), and bioclastic laminae (BCL). There are also four significant lamina combinations (with the increasing TOC values): FQL-CLL combination, FQL-CLL-BCL combination, FQL-CLL-OML combination, and FQL-CAL-CLL-OML combination; (2) differences between laminae occur because of the variability in pore types and structures. There appears to be a greater abundance of intercrystalline pores of clay minerals in the FQL, CAL, BCL, and OML, and well-developed organic pores in the CAL and CLL, and the counterparts of intragranular pores of bioclastic material in the BCL. This detailed characterization provides the following comparative quantification of the thin section porosity of laminae in the second Member of the Kongdian Formation can be differentiated: CAL > FQL > OML > BCL > CLL; (3) differentiation between vertical distributions of laminae is carried out in a single well. The FQL and CLL are widely distributed in all the samples, while the BCL is concentrated in the upper part of the second Member of the Kongdian Formation, and CAL is concentrated in the lower part. This detailed classification method, using geochemical analysis and vertical distribution descriptions, offers a detailed understanding of lamination texture and its effects on reservoir and geochemical properties, which will provide a scientific guidance and technical support to better estimate reservoir quality and to identify new sweet spots in the second Member of the Kongdian Formation in the Cangdong Sag.

Shale oil is an important strategic resource in the future. The development and utilization of shale oil is a pivotal revolution in the field of fossil energy. The successful development in North America brings new thinking to the world. In recent years, there have been many definitions of shale oil, but no agreement has been reached in the petroleum industry. Marine shale oil is dominant in the United States (the U.S.), while continental shale oil is the counterpart in China. This study takes Uinta Basin, Ordos Basin, and Junggar Basin, for example, to make a comparison of the concept and geological characteristics of shale oil in continental basins between the U.S. and China. Different kinds of shale oil are generated due to the distinctions of sedimentary environments, hydrocarbon supply conditions, and sandy sediment development in different regions. High-quality source rocks have been developed in freshwater and salty water environments of continental lacustrine basins. The lithologies of the Uteland Butte Member, Chang 7 Member, and Lucaogou Formation are lacustrine carbonate rocks, clastic rocks, and mixed sedimentary rocks, respectively. Results indicate that the lower pressure can be identified in continental basins in China. Furthermore, the comparison shows that the Chang 7 Member characterizes lighter crude oil and larger scale of “sweet spot” and the Lucaogou Formation developed a thicker target layer, while the Chang 7 Member is at a disadvantage of insufficient stratum energy and the Lucaogou Formation is difficult in heavier crude oil and higher oil viscosity.

The heterogeneity, sedimentary characteristics, and distribution of fine-grained sediments of millimeter-scale laminae were studied in alkaline lacustrine strata in the lower Permian Fengcheng Formation (P₁f₂) in the Mahu Sag to provide a sound theoretical basis for shale oil exploration and development. They were investigated by using core and thin-section observation, geochemical and elemental analysis, mineral identification, and pore examination. Five types of laminae were distinguished, with different mineral compositions, pore characteristics, and distributions. These laminae occur in four distinct combinations. The pore systems predominantly consist of intercrystalline pores, intragranular pores, and dissolution pores of feldspar and dolomite and also contain imporous alkaline mineral particles. Influenced by variations in salinity, the influx of volcanic–hydrothermal material, and the participation of both endogenous and exogenous materials, the formation has gone through five stages of sedimentary evolution. Furthermore, felsic laminae (FQL), dolomite laminae (DOL)/reedmergnerite laminae (RL)/shortite laminae (SL), and chert laminae (CL) developed in single-terrestrial-source still water deposition, hybrid-source still water deposition, and single-intrasource deposition, respectively. Vertically, the FQL-DOL combination shows favorable reservoir characteristics and is the most extensively developed lamina combination in the Fengcheng Formation that primarily developed in the period of the late Feng 1 Member (P₁f₁), the early Feng 2 Member (P₁f₂), the late Feng 2 Member (P₁f₂), and the early Feng 3 Member (P₁f₃). The FQL-RL/SL combination primarily developed in the period of the middle P₁f₂, and the DOL-CL combination is the counterpart in the period of the early P₁f₂ and the late P₁f₂ stages. Considering this in conjunction with the longitudinal distribution of lamina combinations, a model is proposed for the distribution of fine-grained sediments in alkaline lacustrine strata.

The laminae of lacustrine shale in China have been systematically identified and characterized by a combination of core/slice observations, mineral compositions, geochemical analysis, pore structure characterization, and oil-bearing evaluation. The shale of the Chang 7 Member, Yanchang Formation, Ordos Basin was examined as an example in the study. Four types of laminae are developed in the Chang 7 Member, including felsic laminae (FQL), clay laminae (CLL), organic matter laminae (OML), and tuff laminae (TUL). The shale reservoirs exhibit significant heterogeneity. Of these, FQL and TUL have superior reservoir characteristics. The pore diameter of TUL is primarily composed of micrometer-sized secondary pores that are generated during the diagenesis process, while mesopore and macropore development are dominant in FQL. The main source laminae in the Chang 7 Member of the Ordos Basin are the OML and CLL, while the main reservoir laminae are the FQL and TUL. Some of the hydrocarbons produced by hydrocarbon generation are stored in the pore space inside the laminae, while the majority migrate to the inorganic pores of the adjacent FQL and TUL. It confirms that OML and CLL afford abundant shale oil, the combination of organic pores and inorganic pores in FQL and TUL serve as reservoir space, and the “clay generation-siliceous reservoir” shale oil enrichment model is established in the Chang 7 Member of Ordos Basin.

The Permian Fengcheng Formation in the Mahu Sag was deposited in a volcanic-alkaline lacustrine evaporative environment and contains a unique variety of finegrained sediments. This study examines, at a millimeter-scale, the influence of sedimentary microfacies on variability of lamina quality in fine-grained sediments in the second member of the Fengcheng Formation (P1f2). The methods used include thin-section identification, X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), nitrogen adsorption, and nuclear magnetic resonance (NMR). Six types of lamina were identified in two different lithofacies: fan-delta front facies (FDFF) and semideep/deep lacustrine facies (SDDLF). The laminae in FDFF are predominantly feldspar-quartz laminae (FQL), reedmergnerite laminae (RL), shortite laminae (SL), alkaline mineral laminae (AML), and chert laminae (CL). The laminae in SDDLF are predominantly FQL, RL, SL, CL, and dolomite laminae (DOL). Variations in reservoir quality, oil-bearing properties, and the fracability of laminae in different sedimentary facies are determined by the combined effects of lamina density, mineral composition, rock structure, organic matter abundance, and microfractures. Analysis of these factors indicates superior reservoir qualities in FDFF. In SDDLF, the pore structure is limited by high lamina density, chert content, and fine grain size with the NMR porosities of FQL, RL, SL, and CL being 1.32, 0.18, 0.84, and 0.39%, respectively. However, in FDFF, the combination of high organic matter content, feldspar, pyrite, and clay minerals has a superior effect on the organic matter and minerals deposited resulting in better pore structure and more storage space for shale oil. The NMR porosities of FQL, RL, SL, and CL are 2.81, 2.53, 1.80, and 1.12%, respectively. Overall, analysis of lamina variations and their relationships with sedimentary facies indicates that the reservoir in FDFF may offer more favorable targets for "sweet spot" evaluation.