THE SIGNIFICANCE OF SEDIMENTARY CENTER MIGRATION DURING TRANSGRESSIVE SYSTEMS TRACTS IN SHALE OIL AND GAS EXPLORATION IN THE SALT LAKE BASIN: A CASE FROM YINGXIONG RIDGE AREA IN QAIDAM BASIN

Huge non-marine thick clastic-carbonate deposits have developed in Palaeogene-Neogene Qaidam basin in China, and many oil and gas fields have been found in the oil-bearing basin, especially in recent years in shale oil and gas exploration in Yingxiong Ridge area of the western basin. For example, the Oligocene-Miocene N1-N21 layer of Well Cai7 in the deep lake area has both oil and natural gas. Our unconventional oil and gas exploration in the semi-deep lake and deep lake pays more attention to the lacustrine transgressive systems tract. The Qaidam basin is a typical salinized lake basin, and there are three main sedimentary centers. Yingxiong Ridge area is the sedimentary center, but also the main hydrocarbon-generation depression. The deep lake during E32 period developed extensively, and shifted to north-west direction during N1 and N21 period.

The internal interface of N1-N21, which is equivalent to the fourth or fifth level sequence/systems tract boundary, is often a short-term sedimentary discontinuity surface or flooding surface, which mainly controls the migration of the micro-facies zone and the distribution of thin-layer sedimentary sand bodies. Core observation and field outcrop studies found that mudstone and marl developed in the sedimentary center of the lacustrine transgression systems tract (Figure 1 left), with partial lake carbonate rocks (Figure 1 middle). After the sedimentary center migration, the sand body of the flood plain and delta plain developed later can be large and thick fine sandstone and middle sandstone (Figure 1 right). Within the shale and mudstone or lake carbonate rock in two sets of lacustrine transgression systems tract, the development segment of high-quality shale oil and gas sweet spots or the corresponding parasequence or parasequence set is identified, which has become an important target of oil and gas exploration for tight oil and/or shale oil. The study on sequence stratigraphy of high-frequency cycles shows that the evolution of lacustrine transgression systems tract (TST)~highstand systems tract (HST) provides a favorable facies zone for tight oil/shale oil reservoir.

Figure 1: The mudstone and marl developed in the sedimentary center of the lacustrine TST.

Key words: lacustrine transgression systems tract (TST), salinite lake basin, Oligocene-Miocene, shale oil and gas exploration