| In early Hercynian period, epigenic karst, semi open confined karst, and buried confined karst are developed in
the Middle-Lower Ordovician of Tahe Oilfield, Tarim Basin, of which the semi open confined karst is developed in the
shallow coverage zone. There are giant oil and gas resources in karst fracture-cave reservoirs of Middle-Lower
Ordovician in the shallow coverage area of Tahe Oilfield. Confined by the Upper Ordovician Lianglitage Formation and
deep insoluble beds, the fracture-cave reservoirs of Middle-Lower Ordovician in the shallow coverage area show typical
strata-bound distribution characteristics. Karst conditions and karst fracture-cave structure in shallow coverage area are
different from those of exposed karst controlled by drainage base level and buried karst with deep-circulating. In order to
clarify the karst genetic mechanism and fracture-cave structure of Middle-Lower Ordovician of T738 well block in the
Upper Ordovician shallow coverage zone of Tahe Oilfield, the "residual thickness trend combination method" which takes
advantage of both the "residual thickness method" and the "construction trend surface method", was used to restore the
ancient landform and ancient water system of the Early Hercynian, and the insoluble beds and cave beds were identified
by logging and seismic methods. Further more, combined with the dynamic production characteristics of the reservoir, the
transverse-rising fracture-cave structure model under the confined karst conditions in shallow coverage area is
constructed. The results show that: (1) The Early Hercynian paleogeomorphology is generally high in the northeast and
low in the southwest, with shallow-cutting dendritic surface water system developed in NE-SW direction, and the
northern exposed area provides high hydraulic gradient and high-flow karst water supply. (2) T738 well block is in semi
open confined karst conditions, and three sets of insoluble beds (the Chalbach Formation-Lianglitage Formation, the
bottom dense section of the Yijianfang Formation, the lower section of the Yingshan Formation) and two sets of cave
maze beds (the Yijianfang Formation and the upper section of the Yingshan Formation) are developed in Middle-Lower
Ordovician. (3) Influenced by factors such as insoluble beds, hydrodynamic gradients and faults, the cave beds in
Yijianfang Formation and the upper section of Yingshan Formation have longitudinal hydrodynamic relations, and four
types of confined karst fracture-cave structures, including transverse-rising type, isolated type, transverse type and blind
end type, are developed. The research results have important geological guiding significance for the development of
fracture-cave reservoirs of T738 well block in shallow coverage zone of Tahe Oilfield. |
