The Characteristics And Its Control On The Reservoir Development Of The Unconformity Within The Ordovician,Tarim Basin

Ordovician carbonate rocks of the Tarim Basin has become the exploration focus in recent years, and karst reservoir is one of the most important type of the exploration. So the identification method and reservoir formation mechanism of the related unconformities, have become the focus of the study areas. Guided by the theory of sedimentology, sequence stratigraphy, element geochemistry, combined with outcrops, LWD, seismic data, we identified and divide the internal tertiary unconformity as well as secondary interface to Ordovician of the Tarim Basin, built up the mark of the interface identification, and discussed the formation mechanism of the interface and control on carbonate reservoir.Identifications of T78, T76and T72interface are found on the outcrop, drill logging and seismic section. T78and T76interface show up unconformity and micro-angular unconformity through outcrop. According drilling data, limestone deposit upon the interface and dolomite deposit down to it, and in the seismic section, the interface shows as a uniaxial intermittent reflection; in drilling profiles, T72interface is a contact relationshipo of clastic rock/limestone, which shows up as strong reflections with intermediate frequency, continuous, intermediate amplitude in seismic profiling sections. On top of the interface, more characteristics of onlap fill and under the interface truncate characteristics. The property, exposure intensity, and evolution of the interface have been demonstrated. T78interface is judged as a surface composited by a long-term exposure surface and post-transgressive surface. Exposure intensity is the maximum, in the platform margin of Manjiaer Depression western margin in the eastern basin and the platform margin of the southwestern Tarim Basin, decreased along the central platform, and arrived the minimum in the open platform open platform backside the margin of the platform. T72interface is a surface composited by a short-term exposure surface and a transgressive surface, the interface only by the middle Caledonian III screen movement in Central Tarim basin-in Bachu Area of Tarim Basin, through Late Caledonian and Early Hercynian tectonic movement in Southwestern and northern Tarim area, the interface lose partial deletion of the strataMain controlling factors of the unconformity is explored. It is considered that twice long sea-level decreasing within Lower Ordovician is the dominant factor in the formation of the T78, T76and T72interface; T72interface is formed as a drowned unconformity in Lianglitage Group by the short-term sea level at the end of sediment dropping and sea level quickly rising afterward. interface of short-term cycles and platform structural style and sedimentary facies relations are studied. Restricted platform and prograding margin of the platform is considered to be more and lager short-term exposure unconformity, open platform and retrograding platform margin is considered to be less short-term exposure unconformity. Eustatic sea level change within tne platform controls the combination of carbonate microfacies.The reservoir characteristics which are under the control of different levels unconformity surface have been studied. Reservoirs controlled by the tertiary unconformity develop micro karst topography, flat terrain, and a large area of distribution; reservoirs controlled by four-stage cycle interface own a primary porosity and a good foundation of the cave; reservoirs controlled by high-frequency cycle interface have less primary pore.