Oil Correlation for Whole Petroleum System in Qintong Sag, Subei Basin

With the continual progressing of shale oil exploration and development in lacustrine facies, the investigation in Qintong sag of Subei basin steps into the stage of whole petroleum system evaluation. The previous oil correlation researches focused on the influence of salinity of lake water during source rock development onto biomarkers, which is hard to satisfy the requirement of oil correlation with the source rocks of "contemporaneous heterotopic facies" for whole petroleum system. This study is on the basis of analysis of controlling of high-resolution sequence stratigraphy driven by paleoclimate on source rock development and distribution, totally 11 shale oil and conventional crude oil samples in Qintong sag have been employed to measure the aryl isoprenoid biomarkers which can reflect sulfuration habitat and be used to correlate the whole petroleum system with source rocks. Meanwhile, the 2, 3, 6- aryl isoprenoids (2, 3, 6-AIPs) and 2, 3, 4- aryl isoprenoids (2, 3, 4-AIPs) have been applied to build two parameters: C₃=Σ(2, 3, 6-AIPs)/Σ(2, 3, 4-AIPs) and C₄=Σ(2, 3, 6-AIPs)/ Σ(2, 3, 4-AIPs)+ Σ(2, 3, 6-AIPs), which are utilized to calculate the relative sulfuration intensity and relative paleo-depth of lake water for the source rocks of shale oils and conventional crude oils, respectively. And then, several research results have been obtained as following. (1) The second member of Funing Formation in Qintong sag underwent the low frequency cycle of earlier lake expansion system tract (EEST)→later lake expansion system tract (LEST) →high level system tract (HST), which can be divided into high frequency cycle of 9-fourth level quasi-sequence group (P1-P9), and 32.5-fifth level subsequences, which indicates the total tendency from the (semi-) haline water during the earlier cold and dry paleoclimate evolving into the fresh water during the later warm and humid paleoclimate. (2) The relative higher values of residual total organic carbon (TOC) in the second member of Funing Formation are always corresponding to the maximum lake flooding surface of system tract cycles with warm and humid paleo-climate and fresh water; and the relative lower values of residual TOC being always corresponding to the initial lake flooding surface with cold and dry paleo-climate and (semi-) haline water. This leads to a conclusion that bathyal-deep lake environments with warm, humid paleoclimate and fresh water are more beneficial to organic enrichment. (3) Four types of source rocks in the second member of Funing Formation can be recognized: the first is relatively shallower water column and weaker sulfuration facies; the second being relatively deeper water column and moderate sulfuration facies; the third being deeper water column and intensive sulfuration facies; and the fourth being deeper and fresh water column facies. This research is not only providing a new tool for "contemporaneous heterotopic facies" source rock evolution, but also giving the evidence to establish the distribution sequence of whole petroleum system.