Abstract:

The Ordovician carbonate paleokarst cave collapse in the Tahe oilfield is a special karst-type reservoir, and gave rise to the collapse of early caves, many types of wallrock brecciation, as well as certain transformation of pore structure. In this paper, the unique response characteristics of crackle breccia, mosaic breccia, mixed breccia, karst caves and their sediment fillings have been identified by drilling, coring, 3-D seismic data, conventional logging, and FMI image logging. The seismic configuration characteristics of karst collapse and overlying strata are presented, which consist of a downward-concave seismic event and “string” and chaotic seismic reflection. Collapse geometric parameters, such as the expansion index (EI), vertical relief (ΔZ), and the width (W) are used to quantitatively assess the paleokarst collapse. The results indicate that the collapse formation was multi-stage and reached a peak in the Xiabachu mudstone interval. The collapse can be divided into three types through profile analysis: (1)early epidiagenetic-stage karst collapse, which usually develops multilayer cave collapse. Breccias were developed in the collapse zone, forming a complex mixed breccia lithological assemblage supported by limestone matrix, mixed breccia and mud. The reservoir is connected integrally, which has certain storage capacity; (2)the middle-stage (Early Carboniferous) load collapse can be attributed to post-burial collapse, which is composed of cracks, crackle breccia, mosaic chaotic breccia, clastic matrix-supported chaotic breccia and cave-deposited quartz sandstone. There are some unfilled residual caves reservoirs; (3)Late Hercynian fault-controlled collapse, which was formed by coeval tectonic movement and gave rise to a new collapse structure comprising cracks, mosaic breccia, chaotic breccia and filled caves, with better reservoir property. The combination of original cave layers, collapse time and main control factors may have results in the complex reservoir structure of paleokarst cave collapse. It is considered that load collapse and fault-controlled collapse have developed effective reservoirs, which represent favorable targets for oil and gas development.

Key words: karst, collapse, reservoir, Ordovician, Tahe oilfield, Tarim basin