--> --> Depositional and Diagenetic Processes Controlling on the Quality of Tight Sandstone Reservoirs in Middle Permian, Ordos Basin, North China

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Depositional and Diagenetic Processes Controlling on the Quality of Tight Sandstone Reservoirs in Middle Permian, Ordos Basin, North China

Abstract

The Ordos Basin, in areas where faults and folds were not developed, is a monocline with an east-to-west dip of less than 1°. Large braided plain delta systems with shallow water depth and fine-grained sediments were developed in Ordos Basin during Middle Permian. A number of wells have been drilled over the last decade in Sulige gas field, northern basin, focused primarily on gas discovery in the tight sandstones with ultra-low porosity and permeability of Shihezi Formation. Tight sandstones in Shihezi Formation had a detrital mineralogy dominated by quartz, lithic fragment and feldspar, ranging from litharenities to greywacke. Secondary pores were major type of apertures for fluid flows. Sandstone samples, from depths of 3670 to 4120 m, were examined by polarization microscope, electron microprobe, mercury-injection test and X-ray powder diffraction to interpret their diagenetic processes and pores evolution. Braided distributary channels and mouth bars at delta front presented relatively high porosity. The well connected facies of channel and bar sandstone with good preliminary porosity were comprised of relatively coarse and well sorted grains. This property of deposits allowed the subsurface fluids to enhance the subsequent diagenetic processes. The widespread occurrences of quartz overgrowths and linear contacts of grains suggest that Shihezi sandstones underwent strong compaction during the diagenetic history. Feldspars were dissolved by organic acids when the hydrocarbon was generated, and produced calcite and clay minerals (kaolinite, chlorite, and illite) to fill most of the primary pores simultaneously. Secondary porosity developed due to partial or complete dissolution of feldspar grains and calcite cement. Diagenetic processes and pore evolution of the Shihezi sandstones have been established in this study to help better understanding of the tight sandstone reservoirs. Due to the depositional and diagenetic characteristics of the tight sandstone, hydrocarbon explorations need to target the reservoirs which were deposited from braided distributary channels and mouth bars with good connectivity and secondary porosity.