Tight oil sandstones in Upper Triassic Yanchang Formation, Ordos Basin, N. China: Reservoir quality destruction in a closed diagenetic system

An investigation of the Triassic Yanchang Formation, Ordos Basin, N. China, revealed that the diagenesis and quality of tight oil sandstone reservoirs (with an average porosity of 9.83% and an average permeability of 0.96 mD) were controlled by a closed diagenetic system. The carbonate cements observed in the sandstone were derived from decarboxylation of organic matter that occurred in the adjacent mudstones. These reactions supplied CO32-, which reacted with cations derived from grain dissolution in the sandstones. The average size of the diagenetic geochemical system with respect to carbonate cements was small (<6 x 10(-2) m(3)), comprising sandstone and its adjacent mudstone(s). The carbonate cements tend to concentrate in the marginal sandstone, which is taken to indicate that the flux of CO32- into the sandstones limited the quantity of carbonate precipitated. In addition, the mass is near balance between the amount of feldspar dissolution and its by-products in the central sandstone (distance to the sandstone/mudstone interface is mainly more than 1 m), where the permeability of sandstone will present a decreasing trend with the increase of feldspar dissolution pores. The pore space of central sandstone will be just redistributed, with primary intergranular pores converting to feldspar dissolution pores and clay mineral micropores. Thus, the best part of the sandstone reservoirs tends to be the central part of sandstone. In particular, sandstones that are more than 2 m thick could be the potential hydrocarbon reservoirs because they retain the best porosity (average of 13.6%) and permeability (average of 1.8 mD). The results of our study provide an important guide for the exploration of tight oil sandstones in other petroliferous basins over the world.