SEQUENCE STRATIGRAPHY AND SEDIMENTARY FILLING EVOLUTION MODEL OF YANCHANG FORMATION IN SOUTHERN ORDOS BASIN, CHINA

Yanchang Formation developed a continental lacustrine and delta filling in Ordos basin during the Late Triassic, which recorded a complete process of formation, expansion, contraction, extinction of a large freshwater lake. Due to several sets of favorable lacustrine shale source rocks and lacustrine delta sandstone reservoirs develop in internal of Yanchang Formation, it constitutes the most favorable source rocks and reservoir rocks in the basin, belonging to low porosity and low permeability lithologic reservoirs. As a consequence, it is the main oil exploration and development strata in the southern Ordos basin nowadays.

Through comprehensive analysis of seismic data interpretation, rock association and logging data of Yanchang Formation in southern Ordos basin, five sequence boundaries are recognized in Yanchang Formation which can be further divided into 1 first-order sequence(Yanchang Formation), 2 second-order sequences(Chang10-Chang7, Chang6-Chang1), 4 third-order sequences(Chang10-Chang9, Chang8-Chang7, Chang6-Chang4+5, Chang3-Chang1), and the sequence stratigraphic framework is also established.

Based on the analysis of the episodic tectonic evolution, sediments supply, relative change of lake level and sedimentary filling characteristics, sequence filling development model of Yanchang Formation in the study area is established, indicating that there are some differences in depositional system tracts and depositional compositions in the third-order sequence under each episodic tectonic evolution.

The 4 episodic tectonic activities respectively corresponded to the formation, development, transition, decline phase of Ordos basin during Late Triassic, thus there were great differences in the nature of the internal stress and activity intensity between each episodic tectonic activity. On the other hand, the study area located at frontal steep belt of thrust orogenic belt, central depression area and distal forebulge slope belt. Thereby, it is considered that the main controlling factors of the diversities and complexities of sequence architecture pattern and sedimentary filling associations are episodic tectonic activities differentiation and the complicated tectonic distribution patterns of Yanchang Formation in Southern Ordos basin based on the above analysis.