Accumulation of unconventional petroleum resources and their coexistence characteristics in Chang7 shale formations of Ordos Basin in central China

The Ordos Basin is the largest oil and gas producing basin in China, where tight oil, shale gas, oil shale, and other unconventional oil and gas resources have been found in the Chang7 subsection of the Triassic Yanchang Formation. However, the mechanism of formation and the distribution of unconventional oil and gas resources in the shale layers have not been systematically investigated until now. According to the type of unconventional oil and gas resources, main controlling factors, and the maturity, depth and abundance of organic matters, the shale oil and gas resources from Chang7 region are divided into five zones that include an outcrop-shallow oil shale zone, a middle-matured and medium-burial shale oil zone, a medium-matured and medium-burial in situ conversion process (ICP) shale oil zone, a high-maturity and deep-burial shale gas zone, and an adjacent-interbedded tight sandstone oil zone. By the distribution of resources, orderly evolution of oil and gas resources and coexistences in lacustrine shale formations have been put forward, and also a strategy of integrated exploration and development of resources in the shale formations is proposed. Overall, the outcome of this study may guide on the effective utilization of unconventional oil and gas resources in other shale formations.

Characteristics of Diagenesis and Pore Evolution of Volcanic Reservoir: A Case Study of Junggar Basin, Northwest China

Unconventional volcanic reservoir is different from conventional reservoir in reservoir space,diagenesis,pore formation and evolution.The Carboniferous volcanic reservoir was selected in Junggar Basin,Northwest China because based on sediment/rock cores and outcrop data,diagenesis and pore evolution were studied by elemental measurements,thin section observations,and diagenetic analyses.These analyses shows that the reservoir lithology is predominantly intermediate-basic volcanic,and the reservoir storage space is composed mainly of secondary dissolved pores and fractures.The reservoir displays great heterogeneity,and has experienced a great variety of diagenetic alteration during various diagenetic stages including:(1)eruption fragmentation,crystallization differentiation and condensing consolidation at consolidated diagenetic stage;(2)metasomatic alteration,filling,weathering and leaching,dissolution by formation fluids and tectonism at the epigenetic modifications stage.The formation and evolutionary process of the pores is extremely complicated.The primary pores were formed during the consolidated diagenetic stage,and laid a foundation for the late development and alteration of effective reservoir.During the epigenetic modifications stage secondary reservoir storage space was developed via the formation of secondary pores and the development of fractures through weathering and leaching,dissolution by formation fluids and tectonism.

Fluid Mobility Evaluation of Tight Sandstones in Chang 7 Member of Yanchang Formation, Ordos Basin

Fluid mobility has been important topic for unconventional reservoir evaluation.The tight sandstones in Chang 7 Member of the Ordos Basin has been selected to investigate the fluid mobility based on the application of core flooding-NMR combined method and core centrifugation-NMR combined method,and the porous structure is studied using optical microscope,field emission scanning electron microscope(FE-SEM),CT and mercury injection.Our results include:(i)Feldsparrock fragments dissolution pores,calcite dissolution pores,clay mineral dissolution pores,intergranular dissolution expansion pores,inter-granular pores,intra-kaolinite pores,and intra-illite/smectite mixed layer pores are developed in Chang 7 tight sandstones;3D CT pore structure shows that the pore connectivity is positively related to physical properties,and the overall storage space is connected by the throat with diameter between 0.2 and 0.3μm.The percentage of storage space connected by throats with diameter less than 100 nm can reach more than 35%.(ii)Movable fluid saturation of Chang 7 tight sandstones is between 10%and 70%,and movable oil saturation is between 10%and 50%.Movable fluid saturation may cause misunderstanding when used to evaluate fluid mobility,so it is recommended to use movable fluid porosity in the evaluation of fluid mobility.The porosity ranging from 5%to 8%is the inflection point of the fluidity and pore structure.For samples with porosity less than 8%,the movable fluid porosity is generally less than 5%.Moreover,the movable fluid is mainly concentrated in the storage space with a throat diameter of 0.1 to 1μm.For samples with porosity greater than 8%,the porosity of the movable fluid is more than 5%,and the movable fluid is mainly concentrated in the storage space with a throat diameter of 0.2 to 2μm.(iii)The movable fluid saturation measured by core flooding-NMR combined method is generally higher than that measured by core centrifugation-NMR combined method.The former can evaluate the mobility of the oil-water two-phase fluid in samples,while the latter can better reflect the pore structure and directly evaluate the movable fluid in the pore system controlled by different throat diameters.All these results will provide valuable reference for fluid mobility evaluation in tight reservoirs.