Chlorite authigenesis and its impact on reservoir quality in tight sandstone reservoirs of the Triassic Yanchang formation, southwestern Ordos basin, China

Authigenic chlorite plays an important controlling role in reservoir quality, and is widely distributed in the tight sandstone reservoirs of the Triassic Yanchang Formation in the Southwestern Ordos Basin, China. The occurrence and origins of authigenic chlorite were investigated using an integrated approach of optical microscopy, scanning electron microscopy, X-ray diffraction, electron probe microanalysis, and fluid inclusion analysis. The sandstones are fine-grained lithic arkoses and feldspathic litharenite with 11.5% volcanic rock fragments. Authigenic chlorite occurs in two morphotypes, including pore-lining and pore-filling chlorite. Pore-lining chlorite occurs as coatings on the framework grains and covers the entire fining-upward sandstones. Pore-filling chlorite is restricted to the middle to upper part of the fining-upward sandstones and appears as discrete flaky plates or rosettes in the pores or attaches to the surfaces of pore-lining chlorite. Pore-lining chlorite originated from smectite precursors that were derived from volcanic material alteration in the early stage of diagenesis, with temperatures primarily about 90 degrees C. In contrast, pore-filling chlorite was precipitated in pore spaces with higher concentrations of Mg2+ and Fe2+ released from volcanic rock fragments and mica debris dissolution. Moreover, the chemical composition of pore-filling chlorite was affected by early calcite cementation which consumed some ions at temperatures lower than about 110 degrees C. The thin and continuous pore-lining chlorite is the only chlorite type with a positive effect on reservoir quality, whereas the very thick pore-lining chlorite (>6.7 mu m) or porefilling chlorite significantly reduce the porosity and permeability. Systematic studies of authigenic chlorite can provide important insights into the reservoir heterogeneity and are applicable to the exploration and development of tight oil.