|
Calcite is usually the largest cement in hydrocarbon-bearing sandstone reservoirs, which has an important impact on reservoir densification and hydrocarbon accumulation. It is generally believed that calcite cement has multiple sources, phases and diversities, but the understanding is not completely consistent. In this paper, the occurrences and origins of calcite cement are studied in Upper Triassic Yanchang Formation sandstone reservoir in Ordos Basin. Due to the occurrences, the calcite cement could be divided into the disseminated calcite cement and the patchy calcite cement in Yanchang Formation sandstones. Through an integrated research of field investigations, petrographical, mineralogical and geochemical analyses, and fluid inclusion thermometrics, the conclusion is that the origins of the disseminated calcite cement and the patchy calcite cement are same-the released water from adjacent mudstones in the process of compaction, and their present different compositions and contact relations with the clastic particles are caused by their original occurrences. The disseminated calcite cement in the sandstone fills all the intergranular pores so that it could not contact with fluid, leaving no chance for water-rock interaction. While the patchy calcite cement in the sandstone is surrounded by pores so that it can get a chance to contact with acid fluid during hydrocarbon accumulations, which may lead to its in situ dissolution-reprecipitation reaction, the reset of its chemical compositions and the variation of its contact relation with the clastic particles. In view of this, the densification of the sandstones takes place before hydrocarbon accumulation. The conclusions reveal that detailed observations under optical and electron microscope are necessary for further research in calcite origin, which should be the basis for further mineralogical and geochemical analyses as well as fluid inclusion analyses.
|
