Turbidite fan and its controlling effects on hydrocarbon accumulation in the Chang 7 Member of Triassic Yanchang Formation in Ordos Basin

The sedimentary environment of the Chang 7 Member of Triassic Yanchang Formation in Ordos Basin is lacustrine facies with the background of large area mudstone deposition in the deep lake. The turbidity current deposits are widespread, and are char-acterized by extensive sedimentary range, large thickness, and proximal hydrocarbon source rocks. With these favorable factors, the Chang 7 Member has become the advantageous reservoir of oil. It is therefore necessary to find out the characteristics of sedimentary facies of the turbidite fan and sand bodies formed from different microfacies which affected the oil accumulation. Based on the analysis of the field outcrop and the core statistics, the authors divided turbidite fan microfacies in detail, which helps to study the turbidite formation, distribution, and sedimentary characteristics. According to previous research results, the channel turbidite fan with fixed supply and the channel turbidite fan with no fixed feeder were named respectively slope moving turbidite fan and slumping turbidite fan. On the basis of differences in thickness of single sand, turbidite types and formation locations, the authors divided the slope moving turbidite fan into three subfacies, i.e., upper fan, middle fan and the edge of the fan. The three subfacies contain six microfacies, such as the main channel and the overflow deposits. The slumping turbidite fan facies was divided into two subfacies, i.e., central fan and marginal fan. The recognition of microfacies of the turbidite fan and the study of their formation can help the log work to identify favorable sand body, distinguish different oil and gas reservoirs, and control the distribution of oil and gas. Massive texture sand body-and layers sand body help to find the favorable oil and gas reservoir.