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Shale oil reservoirs need fracturing in order to achieve commercial productivity. A key factor influencing the fracturing process is shale fracability, which is strictly dependent on rock fabric. In this study, we focused on the lacustrine shale oil reservoirs located in Dongying Sag and in the Northern Jiangsu Basin. Through X-ray mineral diffraction, rock mechanics, and P- and S-wave velocity tests, we analyzed the mineral components and mechanics of different shale lithofacies, as well as the influence of these factors on shale fracability. Based on the results of these analyses, we proposed a new method for the evaluation of lake shale oil reservoir fracability. The experimental results showed that the macrostructure, mineral content, lithology, and the presence of microfractures in the shale are the main factors affecting its brittleness. Shales with lamellar and sheet-like structures have relatively high Young's modulous or lower Poisson's ratios; under these conditions, netted fractures are easily formed during fracturing. The fracture toughness of carbonate minerals in shales is strong. An appropriate amount of carbonate minerals seemed to increase the shale brittleness; however, an excessive amount increased its cohesiveness, hindering the initiation and propagation of fractures. Felsic minerals are characterized by high brittleness, a characteristic that favors shale fracability; on the other hand, clay minerals and organic matter are highly plastic, a characteristic that hinders fracturing transformations. Due to their macrostructure and composition, lamellar clayey limestones (dolomite), lamellar clayey siltstones, layered calcareous (dolomitic) siltstones, sheet-like clay-containing siltstones, and flaggy carbonate-containing siltstones are expected to be easily fractured. In addition, the presence of microfractures seemed to reduce the overall fracture toughness of the rock: they penetrated artificial fractures, enhancing rock fracability. Taking into account the fracture toughness of carbonate minerals and the plastic effect of organic matter, we proposed a new formula for the calculation of the brittleness index, based on the mineral component method. The brittleness indices obtained through the proposed formula were linearly correlated with those calculated through the rock mechanics parameters method. Overall, our results indicate that the silty lithological successions occurring in the study area should be the easiest to fracture, followed by the calcareous (dolomitic) and clayey or organic-rich lithological successions. This study is of guiding significance for evaluating the fracability of different lithofacies in lacustrine shale oil reservoirs. Especially, the new proposed brittleness index containing the plastic effect of organic matter can correctly evaluate the actual brittleness of organic-rich shale.

The quality of a caprock is closely related to its thickness. Statistical data show that there is an obvious positive correlation between caprock thickness and hydrocarbon column height. This report established that the starting pressure gradient is the key factor in the relationship between caprock thickness and hydrocarbon column height by discussing the concepts and principles of breakthrough pressure, capillary pressure, and starting pressure. Analysis of the physical–chemical properties of the surfaces of mineral particles revealed that the structure of the diffused electric double layer stably develops in the pore throat at the micro- to nanoscale. This is a microcosm of the starting pressure generated in the argillaceous caprock. Based on force analysis, this report establishes that in the process by which fluid breaks through the argillaceous caprock, hydrocarbons slowly displace the formation water in the pore throats and mainly overcome the capillary pressure and adsorption resistance. The quantitative relationship between the caprock thickness and the maximum hydrocarbon column height is determined by considering the starting pressure gradient, and a method is proposed to calculate the maximum hydrocarbon column height of the caprock based on the formation overpressure. This method is successfully applied, and verification via gas testing is performed for offshore oil and gas fields by considering the X gas reservoir. The proposed method has good application prospects in the evaluation of argillaceous caprocks and reservoir risk prediction.

The bedding-parallel fibrous calcite veins in black shales are common in sedimentary basins and retain significant information about shale diagenesis and organic matter evolution. However, the origin of the bedding-parallel fibrous calcite veins in black shales is still of great controversy. Carbonaceous black shales of Es4s–Es3x interval are the main source rock in Dongying Depression, Bohai Bay Basin, and fibrous calcite veins parallel to bedding are widely spread in these shales. Petrographic examination, fluid-inclusion microthermometry, and isotopic analyses were conducted to study the timing, diagenetic fluid conditions and formation mechanism of fibrous calcite veins. The bedding-parallel fibrous calcite veins include beef veins and cone-in-cone structures. The beef veins occur as short lenses and are filled with sub-vertical fibrous calcite. They contain a “median line” defined with brown granular calcite, scattered host rock fragment and pyrite framboids. Several adjacent beef veins may combine and grow together as a cone-in-cone structure and black shale laminas involved in it occur as sinusoidal solid inclusions in slices. Primary two-phase inclusions in fibrous calcite have homogenization temperatures (Th) between 86.4 °C and 117.4 °C. The δ13C composition of the micritic calcite in host shale ranges from +3.7‰ to +6.3‰VPDB and that of granular calcite ranges from −0.2‰ to +1.4‰VPDB, but the fibrous calcite has a moderate δ13C values ranging from +1.8‰ to +5.0‰ VPDB. The δ13C compositions of these calcite suggest an evolving carbon source at different burial stages. The fibrous calcite likely precipitated from modified pore water at elevated temperature, which is supported by its low δ18O values (−13.7‰ to −11.4‰ VPDB). The veins formed over two stage and the fibrous calcite growth was continuous and at least partly driven by the force of crystallization. The bicarbonate responsible for the fibrous calcite was derived from a mixed source including inorganic carbon from previous carbonate dissolution, and organic carbon from both fermentation and thermal decarboxylation.

The Bathonian-Callovian (Middle Jurassic) Dorgali Formation in eastern Sardinia (Italy) consists of dolomitized oolitic grainstone and records a complex diagenetic history. Petrographic examinations revealed three main phases of replacive (D1, D2) and fracture-filling (D3) dolomites with crystal-size ranges of

This paper establishes the sedimentary facies distribution model in the middle of the third member of the Shahejie Formation (Es3z), deep-lacustrine and turbidite fans in the Niuzhuang Sag. Lacustrine turbidite systems can form high-quality oil and gas reservoirs and the understanding of these systems is of great significance for lacustrine basins exploration. We identified a series of turbidite fans by comprehensive analyzing core, well log, 3-D seismic data and grain size variations. These fans can be further divided into lobe axis, lobe fringe, and lobe distal fringe settings. The lobes developed in both directions: the vertical fault strike (east-west) direction and parallel fault strike (northeast-southwest) direction. Porosity and permeability data showed that massive sandstone was mainly developed in the middle lobe axis that has better reservoir physical properties. The paleoclimate was determined using geochemical data. The paleoclimate was warm and humid in the highstand system tract (HST), contributing to the development of the turbidite system.

The subsurface distribution of carbonate cements in sandstones is cornerstone in understanding the fluid regime and origin of cements to shed the light on hydrocarbon migration. Petrography, cuttings, cores, well logs, and seismic data were integrated to better understand the distribution of carbonate cements in the Eocene turbidite-delta sandstones of the Dongying Depression. Petrographic examinations (including cathodoluminescence, CL) reveal that the carbonate cements are mainly calcite with minor ankerite that precipitated in deep burial settings. The significant abundance of carbonate cements near fault zones implies migration of fluids from deep settings through late fractures developed by compaction under deep burial conditions. The extent of carbonate-cemented zones may reach hundreds of meters away from the faults, while scatter carbonate cements are much more extensive. The relationship of pervasive and scattered calcites with oil-bearing sandstones suggests that the pervasive calcite cementation started as soon as the hydrocarbon and brine mixed fluid were displaced while the scatter calcite cements came after. The occurrence of abundant carbonate-cemented zones in the delta front oil-free sandstone intervals suggests possible hydrocarbon migration to the overlying sandstone units.

The origin of fibrous calcite veins developed in organic-rich shales, has attracted great attentions. However the growth direction, formation temperature and stress state are still controversial. The calcite veins in organic-rich shale from lower Es3 to upper Es4 in the Jiyang Depression has been analyzed with electron backscatter diffraction technology (EBSD), which can characterize the microstructures and orientation of mineral crystals in situ. On the basis of core observation and microscopic identification of petrological characteristics, the longitudinal and horizontal profiles of calcite veins were measured by EBSD. Crystallographic information on calcite veins, obtained by pattern quality mapping, phase diagrams, grain distribution maps, Euler graphs, grain misorientations and pole figures, clarifies the origins of these veins. The results show that calcite crystals in calcite veins belong to the sharp-rhombohedral space lattice structure of the trigonal system. Calcite crystals have a preferred orientation on the longitudinal profile, while there is no obvious preferred orientation on the horizontal profile. On the longitudinal profile of calcite veins, the misorientation of calcite crystals is the cause of the interference colour and extinction difference of calcite crystals under orthogonal light. The crystal interference and extinction characteristics on both sides of middle line in calcite veins are different, indicating that the growth direction of calcite veins is syntaxial. A shallow burial environment is suggested for calcite veins initial forming by the evidence that crystal morphology of calcite in calcite veins is most in the shape of sharp-rhombohedron, and some of which have hexagonal prism shape. The average orientation difference of calcite veins on longitudinal and horizontal profiles indicates that the average orientation of calcite grains is inhomogeneous. The stress concentration of small grains is higher than that of large grains, and cracks are developed in the interior or on the border of calcite grains on the longitudinal profile, which indicates that calcite veins are formed in the stress environment of three directional extrusion circumstances.