Petroleum Evolution Within the Northern Tarim Basin, Northwest China: Insights From Organic Geochemistry, Fluid Inclusions, and Re-Os Geochronology of the Deeply Buried Halahatang Oilfield
The chronology of petroleum evolution, especially in Proterozoic–Paleozoic deeply buried petroleum systems, is critical to our understanding of hydrocarbon evolution and the success of exploration programs. The newly discovered deeply buried (>6000m) Halahatang oilfield in the northern Tarim Basin, China, has a potential resource of >70Bbbls. At present, there is no agreement on the chronology of petroleum evolution in this oilfield, with both late Silurian and Permian ages being posited. Combining oil geochemistry, fluid inclusion studies, Re-Os geochronology, and published illite K-Ar dating, we present quantitative chronology of petroleum evolution in the Halahatang oilfield. Similar biomarker features of the oil samples show that the oils are of moderate maturity, biodegraded and represent one oil family, which were derived from a Paleozoic marine source. The homogenization temperatures (Th) of the aqueous inclusions, that are coeval with the hydrocarbon-bearing inclusions, range between 61.2 and 102.3°C (average of 83.2°C). The salinity values are between 8.6 and 15.1 wt. % NaCl eq. Microthermometric data yield modeled fluid trapping temperatures and pressures of 100 to 110°C and ~39 to 59 MPa. This P-T data coupled with previous basin model studies, indicate a single prolonged oil migration event during the Permian. The asphaltene Re and Os abundances of five oil samples vary 0.06-9.47 ppb and 4.9-57.2 ppt. The 187Re/188Os values of the oil range from 78 to 1655 and exhibit a radiogenic 187Os/188Os composition of 1.48 to 4.68. The Re-Os data yield a Model 3 date of 285±48 Ma, with an initial 187Os/188Os composition of ~1.08. The ~285 Ma Re-Os age, which is nominally older than the oil migration/accumulation timing constrained by basin modelling coupled with fluid inclusion analysis and the published reservoir illite K-Ar ages (280–230 Ma), suggest that the Re-Os age represents the timing of oil generation.The Re-Os age indicates that oil generation in the Halahatang oilfield occurred during the Early Permian. Furthermore, basin modelling, fluid inclusion studies, and illite K-Ar dating suggest that subsequent oil migration/accumulation occurred during Mid-Late Permian. This study demonstrates the efficacy of combining crude oil Re-Os isotope chronometry with organic geochemistry and fluid inclusion studies to generate petroleum system models that can be used not only by the petroleum exploration sector in the Tarim Basin but also worldwide.
AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018