Molecular and stable carbon isotopic compositions of crude oils from Shengbei Sag, Tuha Basin: Implications for hydrocarbon Sources, migration and mixing

The molecular compositions of C6-C7 light hydrocarbons, adamantanes, saturated and aromatic hydrocarbons and stable carbon isotopic compositions of individual n-alkanes of crude oils were comprehensively measured to clarify the genesis of crude oils from the Cretaceous and Jurassic reservoirs within the Shengbei Sag of Tuha Basin. The results indicate that crude oils produced from the underlying reservoirs of Middle Jurassic Qiketai Formation are derived mainly from the Middle-Lower Jurassic coal-bearing Shuixigou Group including the coal sequences and interbedded shales. Crude oils generated from the interbedded shales can migrate upward and into the overlying Jurassic and Cretaceous reservoirs and may mix with the low-matured oils generated from the lacustrine shales within the Middle Jurassic Qiketai Formation during secondary migration. Moreover, oil samples from the Upper Jurassic and Lower Cretaceous Formations were altered by gas washing firstly and then gas invasion later by presenting the depletion of n-C12-n-C15 and the enrichment of n-C12-, indicating at least two phases of significant gas generation from the Shuixigou Group. The second phase of intruded gas is supposed to be generated mainly from the Lower Jurassic source rocks in study area. This study provides an improved understanding of hydrocarbon sources, migration and mixing in the Cretaceous and Jurassic reservoirs of Shengbei Sag, Tuha Basin and should be helpful in making reasonable strategy for petroleum exploration in this region.