Kaolinite origins and distinctive influences on deep-buried reservoir: A case study of Pinghu Formation in Xihu Depression, offshore China

Abstract

Substantial hydrocarbons in deep-buried reservoirs are challenged by diagenetically induced heterogeneity, hindering the identification of sweet-spot prospects. Despite being a common diagenetic mineral, genesis of kaolinite is rarely explored from a geochemical perspective, and much less is known about the effects of differential genetic kaolinite on reservoirs. In this paper, the distributary channel of Pinghu Formation in Xihu Sag was selected as a focused object. Petrological and geochemical analyses were conducted, including porosity/permeability test, light/electron microscope observation, electron probe test, and fluids inclusion measurement. In particular, hydrogen/oxygen (H/O) isotopes were applied to determine the genetic mechanisms of kaolinite. The results show that lithology types of distributary channel are mainly lithic arkose and feldspathic litharenite, with quartz comprising 65 %, feldspar sharing 16 %, and fragments sharing 19 % of the total sediments. Despite the uniformity of its detrital components, physical characteristics of the distributary channel exhibit significant variation. Porosity ranges from 3.3 % to 21.4 % (averaging 13.8 %), and permeability ranges from 0.02mD to 614.4mD (averaging 52.1mD). Furthermore, within individual channels, porosity/permeability values are high in the upper sections but fall in the lower. Kaolinite cementation can be observed in both the upper and lower channels, but exhibiting distinctive petrological and geochemical features. In the upper channels, kaolinite is characterized by an embedded-crystal form and low Mg/Ca/Fe content. Based on its high H/O isotopes (averaging −87.9 ‰ δD_-SMOW and 12.3 ‰ δ¹⁸O_-SMOW), the temperature of kaolinite cementation is estimated in the range of 90 °C–110 °C and the calculated δD_water-SMOW/δ¹⁸O_water-SMOW (averaging −90.7 ‰/-11.1 ‰) approached to the organic water region. These features suggest that kaolinite in the upper channels is the by-product of feldspar dissolution by organic acids. High kaolinite content indicates significant feldspar dissolution and extensive secondary dissolved pore space, which is a positive indicator of secondary pore development. In the lower channels, kaolinite is characterized by a sheet-crystal form and high Mg/Ca/Fe content. Based on its low H/O isotopes (averaging −103.8 ‰ δD_-SMOW and 2.0 ‰ δ¹⁸O_-SMOW), the temperature of kaolinite cementation is estimated in the range of 25 °C–50 °C, and the calculated δD_water-SMOW/δ¹⁸O_water-SMOW (averaging −60.5 ‰/-9.4 ‰) indicates a subsurface paleo-fluid environment. These features imply that kaolinite in the lower channels may derive from the recrystallization of muddy fragments. High kaolinite content indicates poor sorting, weak compaction resistance, and low dissolution extent, which negatively impacts both primary pore preservation and secondary pore development. In sum, differential genetic kaolinite may exert opposite influences on reservoirs, and accurately determining their differetial genesis is a prerequisite.