Geochemical characteristics and sedimentary environment of source rocks in the Qiangtang Basin: New discoveries from the upper Triassic Xiaochaka formation in the Woruoshan Mountain

Abstract

Clarifying the sedimentary environment and main controlling factors of source rocks is significant for the oil and gas exploration in the Qiangtang Basin. Based on the detailed observation results, this study analyzed the organic geochemical characteristics and trace metals of source rocks in the Upper Triassic Xiaochaka Formation in the Woruoshan Mountain and investigated the depositional conditions and their implications for the formation of source rocks. The source rocks of the Xiaochaka Formation mainly consist of dark mudstones with low-medium abundance of organic matter. They have type II2-III kerogen and middle thermal maturity. This strata had a warm and humid climate during its deposition. Affected by the sea-level fluctuation, the depositional water columns of this strata evolved from freshwater to brackish water and then freshwater from bottom to top. The Xiaochaka Formation deposited in the oxic water columns overall with high paleoproductivity. The formation of the Xiaochaka source rocks in the Woruoshan mainly is the “productivity model” controlled by paleoclimate and freshwater flux. The warm and humid paleoclimate and freshwater transport contributed to the high paleoproductivity of water columns and counteracted the unfavorable preservation conditions. As a result, type Ⅱ2-Ⅲ source rocks were finally formed.

Keywords

• Qiangtang Basin
• North Qiangtang depression
• Xiaochaka formation
• Sedimentary environment
• Source rock