The difference of sandstone reservoirs with different microfacies in the source and its influence on the development effect -Taking the Chang 71-2 sub-member of Qingcheng Oilfield as an example

Key words: 
• Ordos Basin /
• sublacustrine fan /
• tight sandstone /
• reservoir properties /
• Chang 71-2 sub-member

Abstract: The tight sandstone interlayer of the Chang 71-2 sub-member of the Yanchang Formation in the Qingcheng Oilfield is a typical sublacustrine fan deposit. However, the research on the difference of sandstone reservoir performance on the sedimentary microfacies scale is insufficient, which restricts the screening of high-yield blocks and the difference analysis of horizontal well production effects. Based on core, logging and seismic data, using thin section identification, scanning electron microscopy, micro-CT, nuclear magnetic resonance, high-pressure mercury injection and constant-rate mercury injection tests, combined with oil test and production test data, the differences in tight sandstone rock and ore, microscopic pore structure, oil content and mobility, and oil test in different sedimentary microfacies are analyzed. The results show that under the control of multi-stage slope break zones, multi-stage sublacustrine fan sand bodies accumulate in the relatively gentle semi-deep lake-deep lake slope toe zone, and frequently interact with organic-rich shale to form a large area of ' sandwich ' source-reservoir symbiosis, which constitutes the main geological dessert of interlayer shale oil in Qingcheng Oilfield. The sandstone interlayer can be divided into four sedimentary microfacies : channel, lobe, lobe side edge and slump body. The sandstone superimposed thickness of channel, lobe and lobe side edge is relatively large, and the logging interpretation is generally oil layer, which is the main target of horizontal well development. The reservoir space of channel, lobe body and lobe lateral margin is dominated by dissolution pores and intergranular pores. The cements are mainly ferrodolomite, ferrocalcite and quartz. The channel and lobe body have the characteristics of low content of interstitial materials and cements, high surface porosity, good physical properties and microscopic pore structure, high proportion of connected pore volume, large pore throat radius, and relatively high movable fluid saturation and oiliness. The saturation of movable fluid is closely related to the pore structure. Controlled by the microscopic pore structure, the channel has better microscopic pore throat parameters than the lobe, which is more conducive to the filling and seepage of oil and gas. Under the same thickness scale, the channel is easier to obtain high-yield industrial oil flow than the lobed body vertical well test, and the vertical superposition of the channel and the lobed body is a high-yield sedimentary combination box ; fine sublacustrine fan tight sandstone sedimentary microfacies characterization, optimizing the orientation and length of horizontal wells, and optimizing favorable microfacies combinations for development are the key to improving the drilling rate and development effect of horizontal well reservoirs.