Characteristics and Genesis of Continuous-Tight-Oil Accumulations of the Yanchang Formation (Upper Triassic) in the Ordos Basin, North-Central China

The Upper Triassic oil accumulation in the Ordos Basin is a typical example of a continuous-tight-oil accumulation with reservoir porosity <12% and air permeability <1.0 mD. Continuous-tight-oil accumulation in this basin is characterized by: (1) oil-bearing sandstones are distributed continuously in both the basin slope and the basin center; (2) oil-water relationship is complex and non-Darcy flow dominates fluid flow, without obvious conventional trap boundaries; (3) buoyancy is limited and the pressure difference between source rocks and sandstone reservoirs may provide the driving force for the hydrocarbon migration.

Thin section analysis, fluid inclusion analysis, rock pyrolysis, hydrocarbon simulation experiments, and basin modeling are conducted to discuss genesis of continuous-tight-oil accumulations in this study. Over 750 homogenization temperature data of brine inclusions associated with hydrocarbon inclusions range from 70~125 centigrades, which is identical to the paleotempreture of Early Cretaceous, while Ro is between 0.65~1.0%. Meanwhile, hydrocarbon simulation experiments reveal that excellent source rocks of Chang 7 member generated hydrocarbon continuously during this time and reached its peak intensity at the end of Early Cretaceous. Thin section observation indicates that compaction, calcite cementation, laumontite cementation and quartz overgrowth are major factors causing porosity loss. Light oil’s blue-white fluorescence is observed in secondary porosities, and in calcite and laumontite cements, suggesting that reservoir tightness was at the same time with petroleum charging. Moreover, based on the contents of these major cements and porosity from thin section analysis, it can be deduced that reservoir porosity of Chang 6 member is 9.7~12.7% at the end of Early Cretaceous when it was at maximum depth. Simutaneously, abundant hydrocarbon charging into these tight reservoirs resulted in continuous-tight-oil accumulations. During this process, intense hydrocarbon generation and continuous sand bodies close to the hydrocarbon kitchen are most important. Also, dissolution of feldspar-laumonite caused by intense hydrocarbon generation generates abundant secondary pores and improves reservoir properties, which are favorable for continuous oil accumulations.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California