HYDROGEOMORPHOLOGIC ARCHITECTURE AND DISTRIBUTION CHARACTERISTICS OF EARLIER EPIKARST CARBONATE RESERVOIRS IN MIDDLE-LOWER ORDOVICIAN IN TAZHONG OIL FILED, TARIM BASIN

The Middle-Lower Ordovician carbonate reservoirs in Tazhong Oil filed have been important contributors to oil and gas production for more than two decades. And this layer had progressed to telogenetic diagenensis stage, one of three major stages in limestones as Choquette & Pray(1970) described, when it was exposed to surface. However, it is a little different from the classic definition, because uplift and erosion returns the limestone to the surface immediately and meteoric waters dissolution occurs when it was in the 1^st Phase of the Middle Caledonian, once after sedimentation and without going through the period of burying. Thus, its limestone is soft rock rather than hard rock.

Both the 2D and 3D seismic data indicate that the terrain of the whole Middle-Lower Ordovician is flat and the slope of surface is small. The surface flow pattern has been generally restricted to the paleogeomorphic landform and tends to be diffuse. As a result, erosion and collapse on landform is weak, and it is really impossible to form the drainage system on the surface. Moreover, the flow accumulates when those karren and cracks develop. We conclude it to be "no surface drainage pattern" hydrogeomorphologic architecture.

Karren is the most obvious geomorphic category, including cracks and fissures along some constant directions, where the deepest karst reservoir develops. Meanwhile, we obtained several principal development characteristics of such reservoirs: 1) fissure and fissure-vug reservoir are the broad categories; 2) epikarst reservoir develops universally in horizontal and 3) mainly in step as shallow layers from 30 to 40 milisections in vertical; 4) "short plate" or "beaded" reflection are the primary seismic reflection characteristics; 5) the underlying reservoirs are intimately related to fissures and faults. Those features above reflect a fissure-seepage karst development pattern and is a product of the karst evolution in initiation.