Geological controls on gas content in tidal flats-lagoonal and deltaic shales in the northeastern Ordos basin

Abstract

The gas content of shale is a crucial parameter for calculating resource volume and predicting production, while the key influencing factors of marine to continental transitional shales are still debatable. To reveal the relationship between reservoir characteristics and gas content of tidal flats-lagoonal and deltaic shales, 21 shale cores from the Pennsylvanian Benxi and Taiyuan formations, and Cisuralian Shanxi Formation in the eastern Ordos Basin were taken and analyzed using methods in multiple disciplines. The shales are mainly composed of quartz and clay minerals, with developing silica dominated, clay dominated and mixed lithofacies. The shales are rich in organic matter (TOC averaging 2.43 wt%) and have low gas generation potential (S1 + S2 averaging 2.30 mg HC/g rock). The shales have poor physical properties, with average porosity of 1.7% and permeability of 0.1 mD. Shale gas is mainly organic thermogenic methane with maximum content of 1.44 m3/t and carbon isotope characteristics of δ13C1< δ13C2< δ13C3. The overall distribution of total gas content is positively correlated with TOC content, but the correlation with quartz, porosity, clay, permeability and pore structure is weak or not obvious. The gas content of deltaic shale is mainly affected by the TOC content (R2 of 0.92), while the tidal-lagoonal shale is comprehensively affected by various factors. Thus, the shale gas potential in the transitional environment is controlled by complicated factors, and more targeted work need to be conducted by considering different depositional environments. The results can provide geological basis for shale gas resources evaluation and production optimization in the transitional shale layers.