Control and Import of Sedimentary Microfacies on Tight Oil Occurrence in the Chang 7-3 Member of the Triassic Yanchang Formation, Ordos Basin of China

摘要

The high-resolution microresistivity image logs obtained from 15 wells reveal that the sedimentary microfacies control the distribution of tight oil reservoirs in the Chang 7-3 Member of the Triassic Yanchang formation in the Ordos Basin of China. The microfacies include laminated turbidite, bedded turbidite, and massive turbidite, which suggest different Bouma sequence assemblages of the turbidites formed in semideep and deep lacustrine depositional environments. The tight oil production performance of the Chang 7-3 Member in eight wells further validates that laminated and bedded turbidites of the Bouma sequence most likely control the location of productive tight oil intervals. The organic-rich oil source rock in the Chang 7-3 Member of the Triassic Yanchang formation formed during maximum lacustrine surface flooding in the Ordos Basin. The organic-rich sedimentary material was reworked by contemporaneous turbidity current processes in the semideep and deep lacustrine environment, which resulted in extremely complex lateral variability between oil source rock and different kinds of turbidites and in heterogeneous tight oil reservoirs. Constructing a regional sedimentary microfacies cross section is key to better understanding the tight oil occurrence in the Chang 7-3 Member. Commonly, the gamma ray log provides a distinctive log signature for the individual facies in correlation and reconstruction of stratigraphic cross sections. Herein, the high-resolution microresistivity images provide further details about microfacies sedimentary structures, laminae, massive deposits, slump deformation, slump breccia, and convolute bedding. The microresistivity images also provide the most distinctive image log signature for the Bouma sequence assemblage occuring in the semideep and deep lacustrine environment. This paper demonstrates sedimentary structure identification on the images, sedimentary microfacies and facies association interpretation, and correlation of the stratigraphic cross-section associated with sedimentary structures and oil production performance. This new approach contributes to tight oil reservoir exploration and development because it highlights the influence of sedimentary microfacies on tight oil reservoir distribution in the Chang 7-3 Member of the Triassic Yanchang formation in the Ordos Basin. It will further improve understanding of the widely distributed and heterogeneous tight oil reservoir in the Chang 7-3 Member, and it will also help to reduce the risk and uncertainty of new drilling well locations.