Key Controlling Factors of Physical Properties of Reservoir from Chang 8 Oil Group in the Upper Triassic of Jiyuan Area, Ordos Basin

Jiyuan area is located in the mid-west of the Ordos Basin.Its tectonic units belong to the Yishan slope. Chang 8 oil group mainly develop delta front subfacies which contains underwater distributary channel microfacies,... [ view full abstract ]

Jiyuan area is located in the mid-west of the Ordos Basin.Its tectonic units belong to the Yishan slope. Chang 8 oil group mainly develop delta front subfacies which contains underwater distributary channel microfacies, underwater natural levee microfacies and tributary bay microfacies. Based on the analysis of core, log data,thin section and experiments of scanning electron microscope, mercury-injection,x-ray diffraction, cathodoluminescence as well as acid sensitivity analysis, we hold that Chang 8 reservoir generally has poor physical propertities. It belongs to low porosity and ultra-low permeability reservoir whose porosity is 6%－12%,and permeability 0.1×10－3μm2－1.0×10－3μm2. Pore types are mainly intergranular pores and intragranular dissolve pores. Chang 8 reservoir has small and tiny pores and micro-throats. Average radius from mercur-injection curves is 0.02－0.2μm.Physical properties of Chang 8 reservoir are controlled by sedimentation, diagenesis and tectonism, from which diagenesis plays the leading role.
The lithology is mainly feldspathic litharenite, with a high content of clay matrix（up to 11%）which occupies most of the primary porosity, and inhibits later dissolution. Grain size is mostly 0.1－0.25mm. Sandstone is better sorted and generally rounded, which primarily presents sub-angular to sub-rounded. It has low primary porosity. Physical properties of lower Chang 8 oil group are better than the upper whose sandstones are more pure. In the plane distribution, underwater distributary channel microfacies possesses the best physical properties.
Diagenesis are primarily compaction, cemetation and dissolution.Early compaction contributes most to the destruction of primary porosity, whose reduction rate is about 25%.Cements mainly include clay, carbonate and a few siliceous minerals. Clay mineral mainly presents chlorite rim, whose content is 2%－5%.Chlorite thin films can inhibit the secondary overgrowth of quartz which effectively save the primary porosity. Carbonate cements are widely distributed whose average content is 9%, which have intergrowth texture and usually block pores and throats.Reduction rate of cementation is about 5%.Acid salt reaction showed that hydrochloric acid and mud acid could both effectively improve reservoir properties, during which the former improved better. Acid sensitivity index of hydrochloric acid is from -884597.5 to 0.18, and that of mud acid is form -109999.0 to 0.28. Two stages of dissolution increase the secondary porosity. Dissolution in the middle diagenetic stage A contributed a lot better to the secondary porosity than that in the middle diagenetic stage B. The increasing porosity is about 6%.
Tectonic structures in the study area is relatively stable. Some microfractures can be seen in the thin section, which slightly improves the reservoir.