| Kuqa Depression is an important oil and gas exploration area in Tarim Basin, but the understanding of
Mesozoic petroleum geological conditions of Yangxia Sag in the east is insufficient, which in turn affects the
determination of exploration direction. Based on outcrop, drilling and seismic data, this paper analyzes the petroleum
geological conditions of Yangxia Sag from the aspects of tectonic evolution, stratigraphic distribution, hydrocarbon source
rock conditions and sedimentary reservoir characteristics, etc.
It is believed that Triassic and Jurassic coal measure and lacustrine source rocks are developed in the Middle-Lower
Jurassic in Kuqa Depression, and Yangxia Sag is in the relatively developed area of Jurassic-Triassic source rocks. The
organic carbon content of hydrocarbon source rocks is between 2.1% and 6%, which belongs to high-quality hydrocarbon
source rocks. The CPI value ranges from 1.00 to 1.02, indicating that the hydrocarbon source rock is in the mature-high
mature stage. The Middle Jurassic Kizilenur Formation in Yangxia Sag developed braided river delta downstream plain
facies and front facies sandstone reservoirs, with the main provenance from the southern Tianshan Mountains in the north
and the secondary provenance from Luntai fault uplift in the south. The distributary channel, subaqueous distributary
channel and estuarine bar sand bodies in the downstream plain faces-front facies of braided river delta form high-quality
reservoirs with porosity of 4%-10% and permeability mainly of (1-30)×10-3 μm2, which belong to classⅡ-Ⅲ reservoirs.
The sandstone reservoirs and the coal measure source rocks of the Kizilenur and Yangxia Formations are superimposed on
each other vertically, forming a hydrocarbon-reservoir "sandwich" structure. The sand body pinch out upward to form a
lithological trap in the middle and southern slopes of Yangxia Sag, being overlied by the Cretaceous and Paleogene
mudstone cover, forming a self-generating and self-storing oil and gas reservoir model.
The southern boundary of Yangxia Sag developed a sag-controlled fault during the Jurassic deposition period. Still,
since the Cretaceous to the present, the tectonic activity is weak, which is favorable for oil and gas preservation and
reservoir formation. The rich hydrocarbon shows in the actual wells indicate that the Yangxia Sag has good oil and gas
prospect. A series of lithologic traps formed by southward pinching-out lens-shaped sand bodies in the Kizilenur and
Yangxia Formations were found in the south-central slope zone of the Yangxia Sag, and the comprehensive analysis
suggests that this area is the next key exploration field of the Yangxia Sag. |
