Diagenesis of Upper Triassic Chang 8 Sandstones, Southwestern Ordos Basin, China: A New Insight into the Formation Mechanism of Chlorite and Calcite Cement

Abstract

Authigenic minerals from Chang 8 sandstones can preserve a precise paleotemperature record. Petrographic observations and geochemical characteristics provide a new insight into the formation mechanism of chlorite and calcite and resolve the problems of mobile elements between them. Chlorite is the most widely distributed mineral in the study area. In contrast, kaolinite is absent in some wells and intervals, which has implications for complete chloritization in the sandstones with abundant metamorphic and volcanic rock fragments due to the mixed provenances. Two stages of calcite cement can be identified: (i) early mesogenic, blocky calcite cement (GPI-Ca) precipitated from 84 to 107 °C in the oil-bearing sandstones (δ13CVPDB from −4.8‰ to −17.5‰; δ18OVPDB from −14.5‰ to −17.1‰); and (ii) late mesogenic, poikilotopic calcite cement (GPII-Ca) precipitated from 122 to 137 °C along the sandstone and mudstone (δ13CVPDB from −3.6‰ to −6.8‰; δ18OVPDB from −19.7‰ to −21.6‰). Grain-coating chlorite appeared to form from berthierine precursors from 70 to 110 °C, corresponding to the precipitation temperature of GPI-Ca. Pore-filling chlorite from kaolinite and illite formed at a temperature higher than 125 °C, corresponding to the precipitation temperature of GPII-Ca. GPI-Ca is characterized by nonferroan cement because the isomorphism contributes little at low temperatures and early-stage chloritization suppresses Fe into calcite. GPII-Ca without abundant metamorphic and volcanic rock fragments shows that ferrocalcite coexists with nonferroan calcite, but ferrocalcite does not exist in the sandstones with mafic minerals. Therefore, the chloritization of kaolinite is suitable for more Fe into the octahedral sites of chlorite in the diagenetic system rather than into calcite cement. The results of this study demonstrate the formation mechanism between chlorite and calcite through fluid-rock interactions. Better insights of the diagenetic sequence are significant to reconstruct the pore evolution history.