Imbricate Structure of Fluvial Facies and its Petroleum Geological Significance
Zhong, Jianhua; Shao, Zhufu; Li, Yong; Mao, Cui; Liu, Shengxin; Ni, Liangtian
We have found out that imbricate structure exists in many sedimentary environments such as rivers, deltas, beach, lakes, turbidite rocks and aeolian sand. Imbricate structure not only appears in gravels, but also exists in gritstones, medium sandstones, fine sandstones and even in argillaceous siltstones with different features. We choose Akesai River and Dang River which are in Dunhuang of China as the objects of study. The riverbed lag gravels in the braided channel of Akesai River upstream are mostly igneous rocks and metamorphic rocks. The particles are bulky, at the same time, sorting and grinding is poor. We found that the average dip angle of the particles is 34.47°, the average dip is 155.68°, and the channel direction is 160°-340°through the 107 statistical data. The long axis of gravels is not simple perpendicular to the flow distribution, and in most cases, it has a certain angle with flow. The sorting and grinding of gravels in meandering stream of Akesai River downstream is relatively good. Most of the gravels have particle supported imbricate structure. Imbricate structure tends to upstream at concave bank, and 76 statistical data show that the average dip angle is 35.22°.The maximum flat surface of gravels at convex bank tends to concave bank, and the long axis approximately parallel to river direction, and 90 statistical data show that the dip angle is 32.53°. Another observation point is the edge beach of Dang River downstream. The rocks are mostly conglomeratic medium and fine sandstone, and roundness is good. Imbricate structure in sandstone is more complicated. Firstly, the dip angle changes much, ranging from 12°to 55°; secondly, the dip changes easily, sometimes points to the upstream and sometimes adversely; thirdly, imbricate structure can appear in lamina consisting of single particles, and can also appear in the thin layer containing impurity base as well. Fine grained imbricate structure can form special shielding pores, and make the fluvial facies reservoir have strong heterogeneity. It makes the micro porosity and permeability of three directions be different a lot. The direction which is perpendicular to the dip of imbricate structure is favorable for waterflooding extraction. The discovery of imbricate structure offers guidance for the study of paleocurrent, and makes the fluvial facies configuration to more fine level at the same time. It plays an important role on raising fluvial facies reservoir recovery.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013