Abstract:
The coastal area of the Qikou Sag of Bohai Bay Basin has become an important target for increasing reserves and production in the Dagang Oilfield in recent years. In this area, crude oil from the middle and shallow strata of the region exhibited complex origins, severe mixing, and significant secondary alterations. Multivariate statistical analysis is an efficient, convenient, and accurate data processing method. In recent years, many researchers have applied this method to hydrocarbon source correlation in various regions, yielding fruitful research results. However, its application in source correlation of mixed-source oils requires further development. Guided by petroleum geology and geochemistry theory, this study characterized the geochemical characteristics of crude oils from the middle and shallow strata (the first member of the Shahejie Formation, Dongying Formation, Guantao Formation, and Minghuazhen Formation) and main source rocks (the third member of the Shahejie Formation, the first member of the Shahejie Formation, and the third member of the Dongying Formation). Then, cluster analysis, principal component analysis, and discriminant analysis were applied for oil and source correlation. The results demonstrated that the maturity of source rocks in different strata of the coastal area of the Qikou Sag was relatively similar, and the middle and lower units of the first member of the Shahejie Formation had better hydrocarbon quality. The precursor for hydrocarbon generation of these source rocks was mainly of mixed origin. The middle and upper units of the first member of the Shahejie Formation and the third member of the Dongying Formation source rocks were deposited under weakly reducing, freshwater conditions, while the third member of the Shahejie Formation and the lower unit of the first member of the Shahejie Formation source rocks were formed under reducing, fresh to brackish water conditions. The main secondary alteration in the study area was biodegradation. The crude oils from different strata could be classified into two types: Type Ⅰ crude oil, distributed in the first member of the Shahejie Formation, was characterized by low Pr/Ph values and high tricyclic terpane content. It mainly originated from a mixture of source rocks in the third member of the Shahejie Formation and the middle and lower units of the first member of the Shahejie Formation. Type Ⅱ crude oil was distributed in the Dongying Formation, Guantao Formation, and Minghuazhen Formation. It featured high Pr/Ph values and low tricyclic terpane content, and was mainly originated from mixed source rocks in the upper unit of the first member of the Shahejie Formation and the third member of the Dongying Formation. Discriminant analysis was applied to validate the oil and source correlation results, resulting in an initial classification accuracy of 100% and a cross-validation accuracy of 90.0%. These results indicated that the oil and source correlation results were reliable. The study reveals the promising application potential of multivariate statistical analysis method in source correlation for mixed-source oils.