A significant breakthrough of Permian oil and gas exploration has been made in Duwa Formation of Well Duwa1

located within the Kedong structural belt in the southwestern depression of Tarim Basin. This discovery marks the identification of a new oil-bearing sequence in the southwestern piedmont of Tarim Basin

thereby opening up new prospects for hydrocarbon exploration in this area. Through integrating data from outcrops

shallow drilling

well logging

seismic surveys

laboratory analyses

and regional geological studies

a systematic study was conducted on the hydrocarbon accumulation conditions of the Permian Duwa Formation sandstones around the Kekeya area. The research results show as follows. (1) The Upper Member of Permian Pusige Formation in the periphery of Kekeya area develops semi-deep to deep lacustrine subfacies source rocks. The hydrocarbon generation center covers an area of 1.0×10

4

km

2

with a maximum thickness of 800 m. The measured values of organic matter abundance range from 0.50 % to 2.23 %

and the types of organic matter are predominantly Type Ⅱ and Type Ⅲ. The measured vitrinite reflectance (R

o

) ranges from 0.5 % to 1.8 %

reflecting thermal maturity levels from mature to highly mature stages. (2) During the depositional stage of Duwa Formation

the lacustrine basin in the periphery of Kekeya area experienced shrinkage

thus leading to the development of a sedimentary system that transitions from alluvial fan to braided river

braided river delta

and ultimately to lacustrine deposits

from bottom to top. The shore-shallow lacustrine subfacies are predominant in this region. The reservoirs in the Lower Member of Duwa Formation are primarily composed of underwater distributary channel sands at the braided river delta front

where the single-layer sandbody thickness ranges from 0.89 to 4.2 meters

with a sand-to-shale ratio of 50.72 %. These extensive regional reservoirs are effectively sealed by thick mudstones in the Upper Member of Duwa Formation

forming

an optimal reservoir-caprock combination. (3) The Kekeya and its peripheral area

influenced by multiple tectonic events

has experienced two major phases of tectonic deformation since the deposition of Duwa Formation: the Indosinian and Himalayan orogenies. The Paleozoic formation developed imbricate thrust structures

with the Duwa Formation vertically sandwiched between the Jurassic coal-measure source rocks and the Permian lacustrine source rocks. The Indosinian deformation gave rise to several anticlines and faulted anticlines in the periphery of Kekeya area

which were further uplifted and transformed by compression during the Himalayan period. The central horst zone has served as a long-term migration pathway for hydrocarbons from the Paleozoic to Mesozoic strata

making it a favorable target for oil and gas exploration. (4) In Duwa Formation

the crude oil group composition displays δ

13

C values ranging from -30 ‰ to -28 ‰

suggesting an equivalent maturity of 0.7 % to 1.3 %. The methylphenanthrene index (MPI) of aromatic hydrocarbons in the oil is 0.76

corresponding to an equivalent maturity of 0.84 %. These geochemical characteristics indicate that the Duwa Formation oil is a mature product derived from Permian source rocks

with a maturity similar to that of the underlying Pusige Formation source rocks. This supports a hydrocarbon accumulation model of lower-source and upper-reservoir in Duwa Formation. The Permian Duwa Formation traps in the southwestern piedmont of Tarim Basin are distributed in aligned belts

with four traps identified

covering an area of 85.8 km

2

. The estimated petroleum geological resources are approximately 30 million tons of crude oil and 30 billion cubic meters of natural gas

indicating a promising exploration prospects. These traps are expected to become an important replacement fields for increasing reserve and production in Tarim Basin.