Research On The Sedimentary Facies In Zhenbei Area Of Chang8 Oil-bearing Of Yanchang Formation, Upper Triassic

Zhenbei area is located on the southwest part of Shanbei Slope and the south part of Tianhuan Depression, Ordos Basin. In this area, Chang 8 oil-bearing formation of Yanchang Formation is one of the main oil sequences. Because of the limited research on sedimentary facies, sedimentary system, sandbody distribution and hydrocarbon accumulation, the deep cognizance of oil accumulation was restricted, and no more significant breakthrough was taken. In view of the above problems, on the basis of principles of modern sedimentology, using varied analysis and test, this article studies the stratum, sedimentary source, type and characteristic of sedimentary facies, sedimentary facies distribution and reservoir characteristics, and points out some favorable areas.By methods of marker beds control and cycle comparison, as well as previous stratigraphic classification of Yanchang Formation, Chang 8 oil-bearing strata of this area is divided into Chang 8₂ and Chang 8₁, the relief intensity of which are gentle.According to the research on palaeo-flow direction, light and heavy mineral, cathodeluminescence, geochemistry characteristics and regional geological background, this article believes that, in this area, the sediments of Chang 8 period were from southwest part of the basin. The mother rock were located on Longxi ancient land, which were phyllite and gneiss of Presinian, quartzite of Sinian and epimetamorphic rock and carbonate rock of Cambrian-Ordovician.According to the analysis of outcrops, cores and log facies, braided river deltas are developed in Chang 8 period, Zhenbei area, which can be divided into 3 subfacies and 8 microfacies. Sedimentary facies model is formed. In detail, this article also introduces sedimentary facies and sandbody distributions of Chang 8 period, as well as Chang 8₂ and Chang 81 subperiods. The sandbodys are banded distributed from southwest to north east, which is totally controlled by the distribution of sedimentary facies.The reservoir heterogeneity of Chang 8 period in Zhenbei area is obvious. Porosity and permeability are both lower. The reservoir has the characteristic of middle-low porosity and low permeability. The reservoir property of Chang 8₁ is better than that of Chang 8₂.Through the study of sedimentary facies and reservoir characteristics, combining exploration and development practice and the principles of favorable area prediction, 7 favorable area of Chang 8₁ and Chang 8₂ subperiods are chosen, which provides geological basis of further exploration and development.