UPWELLING DEPOSITS AT THE MARGINAL SLOPE OF A CARBONATE PLATFORM IN QIXIA STAGE AND ITS RELATION WITH HYDROCARBON SOURCE ROCKS

The Qixia Formation is one of the four suites of hydrocarbon source rocks in south China. Through field survey, and organic geochemical and paleontological studies, the authors believe that the Liangshan Member of the Formation is consisted of littoral mash and lagoonal facies; the Stink Limestone belongs to the transgressive carbonate slope facies; and all of the Upper and Lower Silicious Deposits, the Major Limestone and Top Limestone are upwelling slope facies deposited on the edge of a platform, in which storm deposits are observed. The upwelling slope facies could be divided into four sub-facies. Data suggest that it is not appropriate to set up a lower threshold of TOC for marine carbonate hydrocarbon source rocks. The threshold in fact depends upon the age and exposure time of source rocks. The organic matter in the Qixia carbonate is of high quality and dominated by sapropel. The original capacity of hydrocarbon generation is high. The source rocks have passed into a mature or over-mature stage. In the source rocks, the content of micritic carbonate is positively correlated with TOC. High-quality source rocks occur on the top of the Stink Limestone, middle and upper parts of the Major Limestone and some parts of the Top Limestone.