Origin and mechanism of the formation of the low-oil-saturation Moxizhuang field, Junggar Basin, China: Implication for petroleum exploration in basins having complex histories

The Moxizhuang field is a small oil field in the central Junggar Basin containing several low-saturation, low-resistivity oil reservoirs, which display a complex correlation between oil saturation and porosity and permeability that is atypical of both the filled and drained reservoirs. Biomarker associations of crude oil and grains containing oil inclusions (GOIs) of both the present-day water-bearing zones (water zones) and the oil- and water-bearing zones (low-oil-saturation pay zones) were analyzed to investigate the mechanisms for the formation of the low-saturation, low-resistivity oil accumulations. The biomarker assemblage and hierarchical cluster analysis indicate that oil in the Moxizhuang field was mostly generated from Permian source rock deposited in brackish to hypersaline anoxic environments. The pay zones and several water zones display GOT values as much as 38%, greater than the generally accepted threshold GOI value for an oil column (5%). These GOT values are similar to those for high-saturation oil reservoirs in the Bohai Bay Basin and oil zone samples from Australian basins, suggesting that both pay zones and water zones were high-saturation oil reservoirs in the geologic past. Geologic history analysis shows that the Moxizhuang field was located on the north wing of a paleoanticline during oil charge in the Late Cretaceous to Paleogene. This anticline has gradually evolved into a south-dipping monocline since the Neogene, causing northward remigration of accumulated oil. Differential lateral leakage of accumulated oil in different sandstone layers during the remigration phase led to the formation of the water zones with high GOI values (completely drained reservoirs) and the low-saturation, low-resistivity pay zones (partially preserved reservoirs) and caused the complex correlation between oil saturation and porosity and permeability. Compared with other postaccumulation physicochemical alteration processes, lateral leakage has rarely been recognized. Recognizing differential lateral leakage of accumulated petroleum with the existence of high-quality unfaulted cap rocks has important implication for petroleum exploration in sedimentary basins having complex evolution histories.