Evaluation of gas-rich “sweet-spot” and controlling factors of gas–water distribution in tight sandstone gas provinces: An example from the Permian He8 Member in Sulige Gas Province, central Ordos Basin, Northern China

Abstract

The objective optimization of the gas-rich “sweet-spot” is the key to efficient exploration of tight sandstone gas reservoirs. However, research on evaluating the gas-rich “sweet-spot” based on the controlling factors and models of gas–water distribution of tight sandstone is limited. The sand bodies of the Permian He8 Member in Sulige Gas Province of the Ordos Basin are fluvial-delta distributary channels; the gas reservoir porosity dominantly ranges from 6% to 12%, with an average of 8.78%, and exhibits the permeability of (0.1–1) × 10–3 μm2, with a median of 0.38 × 10–3 μm2. The gas reservoirs are tight lithologic lens type with no edge and bottom water, showing a large-area quasi-continuous distribution with horizontal contiguous and vertical multi-layer superposition. Formation water related to the reservoir accumulation process is classified into paleo-formation water, paleo-condensate water, and present-condensate water. The controlling factors of gas production via the source rock and reservoir conditions exhibit distinct stages. The relationships between gas production and the source rock Ro, HGI, reservoir porosity, and permeability are positive, when gas production is <1 × 104 m3/d. However, it can be controlled by a combination of factors when gas production is >1 × 104 m3/d. Formation water production is related to a low HGI, poor physical reservoir properties, large sandstone thickness, excessive fracture development, and gaseous water condensation. Three gas–water distribution models of the gas-rich, moderate gas-rich, and complex gas–water types were established because of the accumulation conditions and gas test results in different areas. The gas-rich type (water-poor gas reservoir) is represented by the Suzhong, Sudong-I and Sudong-III areas, moderately gas-rich type (water-less gas reservoir) is distributed in the Sunan area, and gas–water complex type (water-rich gas reservoir) is mainly distributed in the Suxi-I, Suxi-II, and Sudong-II areas. Based on the analyses of the high weighted parameters of the controlling factors, the values of the gas reservoir comprehensive evaluation index (F) for commercial gas well area, low production gas well area, and gas-containing well area were >31, 26–31, and <26, respectively.