PRELIMINARY RESULTS ON HETEROGENEITY OF LACUSTRINE SANDSTONE RESERVOIRS AND ITS DEPOSITIONAL AND DIAGENETIC CONTROLS, CHANG-6 UNIT OF UPPER TRIASSIC YANCHANG FORMATION, ORDOS BASIN, CHINA

Heterogeneity of sandstone reservoirs occurs at multiple scales and is dynamic due to episodic water-rock interactions and oil migration during burial. It is controlled by primary compositional, textural, and structural heterogeneity and diagenetic alterations. Exposed sandstones of the Upper Triassic Yanchang Formation in Ordos Basin are studied to assess the impact of heterogeneity on oil migration at m-mm scales from depositional sequences to laminae. The section is 20 m thick, composed mainly of sandstones. High-resolution outcrop surface models constructed using data acquired by an unmanned aerial vehicle display 3-D variations of stratigraphic surfaces and units at a lamina scale. The entire section was continuously sampled using a rock saw. It contains 4 upward-coarsening successions of very fine to very coarse arenites and subarenites deposited in lacustrine sublittoral to beach environments and capped by thin paleosols. They are well to very well sorted and subrounded to rounded. Feldspar and quartz dominate with common lithics, biotite, and plant remains. Various tabular and trough x-beddings, plane beds, and internal truncational and reactivation surfaces are well developed, suggesting a wave-dominated setting. Common Skolithos and rare subvertical fractures are present. Oil occurs as ancient solid bitumen and modern seeps, indicating at least two episodes of migration. The bitumen occurs extensively in all sandstones, regardless of texture and composition, along bedding planes as continuous sheets, in burrows and fractures, and as cm-size blobs. Microscopically, bitumen lines pores, encircles blocky calcite cements, and is concentrated randomly along bedding planes. The modern liquid oil seeps have a sharp or diffuse front and are variably confined by bedding planes and fractures. Overall, the occurrence of both solid and liquid oils appears to be partially controlled by depositional texture and structure but, in other cases, random. A non-dimensional index is needed to characterize the multi-order textural heterogeneity, which can be related to reservoir physical properties. Finally, numerical modeling of oil migration in such a framework will provide insights into oil migration and accumulation in heterogeneous sandstone reservoirs.