Shiyou shiyan dizhi (Jan 2024)
Geochemical characteristics of crude oil and contributions to hydrocarbon accumulation in multiple stages in Tahe subsalt area
Abstract
The process of hydrocarbon accumulation in polycyclic superimposed basins often involves multistage crude oil charging, but not every stage of crude oil charging plays a significant role in the current hydrocarbon accumulation. Fluid inclusion methods can only provide constraints on the stages and time of crude oil charging, but cannot determine the contribution of each stage of crude oil charging for hydrocarbon accumulation. To study the contributions of different stages of crude oil charging for the reservoirs, the Ordovician carbonate reservoirs in the subsalt area, Tahe oilfield (Tahe subsalt area) were taken as an example. This study employed crude oil geochemistry, fluid inclusion analysis, fluorescence spectrum, and single well simulation analysis methods to systematically reveal the fluorescence characteristics of crude oil and hydrocarbon accumulation stages and time of the study area. The analysis of crude oil biomarker parameters indicates that the crude oil has the same parent material and was deposited in a marine weak reducing environment. By comparing the crude oil with the study area's source rocks, it was determined that all the crude oil was derived from the source rocks of the Lower Cambrian Yuertusi Formation. The aromatic methylphenanthrene index and methyldibenzothiophene parameters of crude oil serve as effective indicators for the quantitative evaluation of crude oil maturity. The calculated crude oil maturity (Ro) in the Tahe subsalt area ranges from 0.90% to 1.47%, and this may correspond to multiple stages of oil and gas charging. Based on fluorescence spectrum analysis of oil inclusions and homogeneity temperature and salinity measurements of associated saline inclusions in the Ordovician reservoirs in the Tahe subsalt area and the study area's burial history of single well burial history and thermal history simulation, it was determined that the oil reservoirs in the Tahe subsalt area experienced three stages of crude oil charging during the stages of middle Caledonian (420 Ma), middle Hercynian (318 Ma) and late Himalayan (10 Ma). By comparing the fluorescence spectrum parameters of crude oil and oil inclusions in these three stages, it is concluded that the late Himalayan is the primary stage of hydrocarbon accumulation in the Tahe subsalt area, making the largest contribution for the Ordovician reservoirs in the Tahe subsalt area.
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