PRELIMINARY RESULTS ON CHARACTERIZING STRATIGRAPHIC, COMPOSITIONAL, AND TEXTURAL HETEROGENEITY OF LACUSTRINE SANDSTONE RESERVOIRS, UPPER TRIASSIC YANCHANG FORMATION, ORDOS BASIN, CHINA

Sandstones are multi-component physical systems and contain domains and bounding surfaces of variable compositional and spatial attributes, which can be used to define compositional and spatial heterogeneities. Statistical parameters of a specific attribute can be used to quantify the degree of heterogeneity. Heterogeneity is multi-ordered and may be scale invariant. These hypotheses are tested using outcrop and petrographic data of lacustrine sandstones of Upper Triassic Yanchang Formation in Ordos Basin, China. The 20-m thick section contains three wave-dominated deltaic and shoreface sequences bounded by boundaries of sequences, systems tracts, depositional systems, beds, and laminae. Number of stratigraphic surfaces and lithofacies per unit length and area along the section and on drone-acquired surface models show stratigraphic heterogeneity of type and thickness of sandstones, which is most severe around sequence boundaries. Variations of grain size and composition of very fine to very coarse arenites and subarenites are documented by a sliding grid on thin sections. Feldspar and quartz dominate with common lithics, biotite, and plant remains. The kurtosis of quartz abundance per unit grid area is an indicator of compositional heterogeneity. A flat platykurtic distribution suggests an even distribution of quartz abundance and, thus, lower degree of heterogeneity. Both the standard deviation and kurtosis of grain size distribution reflect textural heterogeneity. Preliminary results suggest that quantification of stratigraphic, textural, and compositional heterogeneities of sandstones is difficult but achievable. Finally, oil occurs as ancient bitumen and modern seeps in the studied sandstones. Ongoing and future studies aim to find a non-dimensional index to characterize multi-order sandstone heterogeneity and to relate it to oil occurrence and abundance.