An approach to the source rocks from the Upper Triassic Yanchang Formation in the Majiatan region on the western margin of the Ordos Basin

The deep lacustrine shales occur mostly in the interior of the Ordos Basin. However, there remains uncertain about how and whether they occur or have the hydrocarbon potential in the thrust zones on the western margin of the ordos Basin. The present paper gives a detailed study of thickness, horizon, distribution and hydrocarbon potential of the black shales from the Yanchang Formation on the basis of outcrops and seismic data from the Yitan-1 well and other wells in the Shigouyi geosyncline. The results of well logs, seismic horizon tracing, seismic forward modeling and pyrolytic analysis of the source rocks led us to draw the following conclusions. (1) There are still 2 to 4 beds of the black source rocks in the Yanchang Formation, Majiatan region in the central part of the thrust zone east of Qingtongxia-Guyuan fault. These black source rocks are characterized by high gamma, high time difference and low-resistivity well logs, 3 to 5 m in thickness for a single bed with a total thickness of about 20 m in the Chang-10, Chang-9, Chang-8 and Chang-7 horizons, and also with high amplitude responses. (2) The thin-bedded black shales in the Chang-10 horizon at the bottom of the Yanchang Formation in the Majiatan region are interpreted as the indicator for the distinction from the Upper and Middle Triassic strata. (3) The deep and shallow parts of the lake exhibit the "cross" distribution, isolated and discontinous with the main body of the lake basin. The Chang-10 and Chang-9 horizons are believed to be major source rocks. (4) The samples collected from the Yitan-1 well for the pyrolytic analysis indicate better to good source rocks with good hydrocarbon potential.