Petroleum genetic types and oil–gas sources of deep reservoirs in Baxian Depression, Northeast China

• Gases from ND 1 and WG 3 are condensate-associated gases; XL 1 is oil-associated.
• The maturity of Es4 is higher than that of Es3 and source materials are different.
• Oil source discussion shows that the deep crude oils derived from Es4 Formation.
• Deep source rocks (Es4 or deeper) have great potential of hydrocarbons generation.

Deep oil and gas resources are increasingly becoming more important as the exploration potential of shallow resources faces ever growing challenges after many years of research and production. The discovery of Well Niudong 1, with a high production rate of 642.9 m3/d of oil and 56.3×104 m3/d of gas, is the deepest reservoir (over 6000 m in depth) found in East China. In order to fully understand the genetic types and probable origins of these crude oils with deep burial depth from Well Niudong 1 and 4 other adjacent deep wells (Wenan 1, Wengu 3, Xinglong 1 and Xinjia 4) in Baxian Depression, a variety of biomarker and non-biomarker parameters were discussed using gas chromatography (GC), gas chromatography-mass spectrometry (GC–MS) and stable carbon isotope techniques. In addition, over 100 samples from potential source rock candidates were submitted for detailed geochemical analysis. Thus, enormous geochemical parameters, referring to maturity, organic matter type and environmental facies, such as n-alkanes, steranes and hopances, were obtained. Deep crude oils from Baxian Depression display pristan/phytane (Pr/Ph) ratios from 2.06 to 3.35 and low sulfur content. Based on the relationship between δ13C1 and C1/C2+3, gas from Niudong 1 and Wengu 3 Wells are both condensate-associated gases and the gas from Well Xinglong 1 is oil-associated gas. The geochemical parameters show that source rocks from Es3 and Es4 formations are deposited in weak reducing and slight oxic environment, the water body is brackish and relatively clean, respectively, the maturity of Es3 source rocks are from low mature to mature, while that of Es4 is from mature to high mature. Geochemical characteristics of crude oils with deep burial depth show some similarities to those obtained by the analyses of the Es4 source rocks in terms of thermal maturity, organic matter input and facies. Therefore, we suggest that the Es4 has acted as the most probable source rock candidate for generating these oils. With the great thickness and wide distribution of Es4 in Baxian Depression, an enormous potentiality of deep oil and gas exploration in this area is expected.