An organic geochemical investigation of oils and source rocks from two Mesozoic formations of Shanganning Basin, China

Abstract

Source rocks and crude oils from two Mesozoic Formations, the Upper Triassic Yanchang Formation and the Lower Jurassic Yanan Formation of the Shanganning Basin, China, have been studied and characterized by a variety of geochemical techniques. The major aims of the study were to characterize the source rocks and depositional environments of the basin and attempt oil/source rock correlations to improve our understanding of the origin of oil and gas in this basin. The black shales of the Yanchang Formation were deposited in a lacustrine environment of varying water depths and dominated by aquatic organic matter with minor contributions of higher plants. The shales are rich in organic matter and of marginal to peak maturity. The source rocks of the Yanan Formation were deposited in shallow lake and fluvial swamp environments and contain interbedded coal layers and the organic richness of this formation is less than that of the Yanchang Formation. The Yanan Formation also has a lower maturity level and more higher plant input than the Yanchang Formation, and would therefore be expected to make a minor contribution to the crude oils found in the basin and is proposed to be more gas prone than oil prone. Oil/source rock correlation studies show that the crude oils found in both formations were produced predominantly from the underlying Yanchang Formation.