Abstract:

In order to point out a favorable area for exploring petroleum of the Upper Triassic (the yenchang Series) in the Shan-Gan-Ning Basin, we have studied the depositional environment of the compact reservoir sandstones in the midst of the yenchang Series, their characteristics of diagenesis and the factors that have affected petroleum accumulation. Having surveyed 7 sections and the cores of over 20 wells, observed many thin—sections of rocks with polariscope and pluoroscope and analysed various relevant documents, we have come to the conclusion that the middle period of deposition of the Yenchang Serics was the time when most Basin widely advanced the lake in the Upper Triassic of the Shaw-Gaa-Niag.Then the deep lake basin developed about 40000 KMZ in the SouthVTest of the basin. In it were deposited turbidite fan sandstone and deep lake mudstone. The western margin of the basin was close to the source regions and very steep, so that pebbly subaquatic alluvial fans were forrited that were deposited from agravity flow. There was a gentle slope in the East of the basin, and the sediments majorly belong to sandy delta.The main reservoir sandstone of the middle section of the Yeachang Series are the turbidite fan sandstones. They are poorly porous and sorted, Under compaction the pores were reduced quite fast and the remained pores are mainly irreducible which are less than 10 microns. This turbidite fan sandstone, close to the source rocks，was favored in receiving the primarily migrating petroleum and forming reservoirs, whose porosity and permeability are very low.At the end of the paper it is suggested that the west area of the San-Gan-Ning Basin be the favorable place for present petroleum exploration of the Yenchang Series because in this area the coarse detrital sediments are developed, they are close to the Bourse rocks and there were some tectonisms that had—great influence on petroleum accumulation.