Sedimentary record of late Mesozoic extension, southeast Mongolia: Implications for the petroleum potential and tectonic evolution of the China-Mongolia border region

Sedimentary and volcanic fill of the East Gobi basin in southeastern Mongolia records Late Jurassic-Early Cretaceous northwest-to-southeast-oriented extension. At least four synrift sequences are observed in outcrop and on seismic reflection profiles, with more than two km of basin fill preserved in the subsurface. 40Ar/39Ar ages on mineral separates from volcanic units indicate rifting from at least 155–125 Ma. Basin inversion occurred during the middle Cretaceous (approximately 110–100 Ma), followed by deposition of Upper Cretaceous postrift strata. Fluvial-deltaic synrift facies form the main hydrocarbon reservoir units in the East Gobi basin, as seen in outcrop and in core. Reservoir quality is limited by laterally discontinuous channelized sand beds, lithic-rich sandstone compositions, and the presence of porosity-reducing zeolite cements such as analcime. However, repeated cycles of lake expansion and contraction resulted in the possibility of multiple stacked deltaic targets compartmentalized by basal transgressive shale units as seen at the Har Hotol locality. Molecular organic geochemical parameters indicate that a widely deposited Lower Cretaceous lacustrine mudstone unit observed in core is the most likely source-rock for oil in the basin. Oil and source-rock extracts demonstrate geochemical evidence for significant algal input in stratified lacustrine source environments. Biomarker data from oil samples of the Zuunbayan and Tsagan Els fields also show differences in the degree of algal input into source facies, as well as variations in maturity. Basin structure in the northeastern part of the basin is mainly controlled by high-angle faults, as observed along seismic reflection profiles. However, an Early Cretaceous metamorphic core complex on the southern edge of the basin indicates that the East Gobi was partitioned into regions of high and low strain extension. Mechanisms driving regional Late Mesozoic rifting in eastern China and Mongolia have traditionally been ascribed to backarc extension, but now are expanded to include the possibility of gravitational collapse of orogenic belts, particularly along former suture zones, as well as strike-slip transtension. Collisions with the Siberian craton and the Lhasa block to the north and south of Mongolia, respectively, may have driven intracontinental deformation during rift time.