Discovery and geochemical characteristics of Chang 7 source rocks from the eastern margin of a Triassic lacustrine basin in the Ordos Basin

Abstract

Previous studies of source rocks in the Triassic Yanchang Formation in the Ordos Basin were mainly focused on the inner part of the basin, especially on the depocenter and its surrounding areas. Few investigations have been made with regard to source rocks on the "margin" of the lacustrine basin, particularly on the eastern margin. In order to make up for the limitations of previous work and determine whether effective source rocks are developed on the eastern margin of the Ordos Basin, we chose the Qilicun Oilfield in Yanchang County as our study area. The characteristics of the Chang 7 source rocks from the Qilicun Oilfield were studied through core observations, logging curve analyses and geochemical tests. The Chang 7 source rocks in the Qilicun Oilfield were widespread, with an area of over 2 500 km2. They were subdivided into two types: black shale and dark mudstone, averaging 9.2 m and 28.9 m thick, respectively. The average TOC content of the black shale is 2.73%, which is in accordance with the "best" source rock standard. The average TOC content of the dark mudstone is 1.98%, which falls into the "good" source rock category. The organic matter is of types Ⅰ-Ⅱ1, mainly generating oils. Maturity parameters show that there is no significant difference between the black shale and the dark mudstone, and both have entered the main oil generation stage. Biomarker analyses indicate that the Chang 7 source rocks were deposited in continental fresh water in a reducing environment. The black shale depositional environment was more reducing than that of the dark mudstone. The source organic matter is mainly derived from lower ranked aquatic organisms, with some higher land plants. Compared with the black shale, the dark mudstone has more contribution from terrestrial higher plants. The Chang 7 source rocks from the Qilicun Oilfield on the eastern margin of the Ordos Basin is the major source rock for hydrocarbon accumulation in this area. Moreover, the discovery of good quality source rocks suggests that the Chang 7 section on the eastern margin of the Ordos Basin such as the Qilicun field has significant potential for shale oil/tight oil exploration.

Keywords

• source rock
• geochemical characteristics
• yanchang formation
• qilicun oilfield
• ordos basin