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Integrated Geological and Geochemical Constraints for Petroleum Fluid Charge Models in the Circum-Manjiaer Region, Tarim Basin, NW China

Abstract

This study describes new geochemical technologies and workflows, to provide integrated geological and geochemical constraints for the models and timing of petroleum fluid charges in the Lower Paleozoic petroleum systems in the Tarim Basin, NW China. Following the recent discovery of the Shunbei oilfield by Sinopec, the presence of several united giant petroleum plays along the Lower Ordovician carbonate platform margin in the circum-Manjiaer region has been confirmed. Conventional GC and GC-MS analysis of saturated and aromatic hydrocarbon fractions reveal at least two oil families in the study area, one carbonate rich, another more clay minerals rich. The presence of a carbonate source rock for the oils is further supported by thiophenic aromatic biomarkers recognized by FT-ICR MS analysis. As many of the heavy oils and solid bitumens in the study area have undergone different extents of biodegradation, we adopted a mild oxidation method to release the occluded molecular marker compounds from petroleum asphaltenes. The occluded compounds from the main Ordovician reservoired oils of the Tahe oilfield display an early oil window maturity and high gammacerane contents, indicative of an early formed oil from source rocks deposited in a saline and highly stratified environment. The study of hydrocarbon charge through geological time and calibration to present day fluid observations has been integrated with stable carbon isotopes, aqueous and hydrocarbon inclusions, utilizing our own newly patented technologies. Our data provide credence for the three-phase oil charge model proposed for the Lower Paleozoic petroleum systems in the circum-Manjiaer region of the Tarim Basin. In the Tahe oilfield, for example, the initial oil charge to the current Tahe location came from the local evaporitic source in the Cambrian; this was followed by oils from a carbonate rich source to the south and east of the oilfield; and more recently by fresh light oils generated from further south and east.