Methods to identify near-source hydrocarbon-bearing potential reservoir using geophysical well logging data: A case study of the Triassic Chang 8 Formation in Pengyang Region, Southwestern Ordos Basin

The Triassic Chang 8 Formation in Pengyang Region, Southwestern Ordos Basin, belonged to typical near-source hydrocarbon-bearing reservoirs; the hydrocarbon accumulation mode did not meet the classical law. This led to low resistivity contrast between hydrocarbon-bearing reservoirs and water saturated layers, and the common methods of identifying hydrocarbon-bearing potential reservoirs based on geophysical well logging data lose their role. In this study, based on the analysis of petrophysical experiments and geological structure, we concluded that the main reason that led to low resistivity contrast in the Chang 8 Formation was high irreducible water saturation ( S wirr ), and the key factors that controlled pore fluid distribution contained the effective thickness of source rock, the straight-line distance between effective source rock and the top depth of Chang 8 Member, and the reservoir quality. Hydrocarbon-bearing formations always contained good quality, short straight-line distance between Chang 8 Member and effective source rock and corresponded to thicker source rock, and vice versa. We defined three parameters of RQI, H1 and H2 to express the quality of Chang 8 Member, the effective thickness of source rock of Chang 7 Member and the straight-line distance between Chang 8 and Chang 7 Members, and a combined factor of (Rt × RQI) × H2/H1 was raised to identify near-source hydrocarbon-bearing potential reservoirs. Meanwhile, the crossplot of water saturation ( S w ) and free water saturation ( S wf ) was also proposed to distinguish low resistivity contrast near-source hydrocarbon-bearing potential reservoirs and water saturated layers. Comparing with common methods, the identification accuracy of near-source hydrocarbon-bearing potential reservoirs in the Chang 8 Formation increased from 68.75% to 93.75%.