Patterns And Structural Positions of Productive Fractures In the Asmari Reservoirs, Southwest Iran

Abstract

Abstract In wrench faulted areas, similar to the one introduced here, identifying structural styles and their histories could be one of the most important tasks in studying tectonic related fractures and faults. The main purpose of this paper is to examine the patterns and structural positions of productive fractures in different reservoir structures of the Asmari formation (Oligo-Miocene) in the Khuzistan oil fields belt of southwest Iran. Details of productive fracture positions have been studied on four major Asmari reservoirs, using:production data (e.g., production rates and productivity indices) which are the most reliable indirect fracture identifiersmud losses, to identify open fracture portionfaulted well assemblages to find tectonically active Portionssecond derivative map to find the portions of the maximum rate-of-change-of-dip on top of the reservoir structures, where it is expected to have maximum fracture density. The results were then compared with the surface fracture patterns and densities in the mountain front boundary in the area. In general, productive fractures were found to be associated mainly with the plunges and bends of the Asmari reservoir structures. To assist in determining optimal horizontal well trajectories in the anticlinal traps of the studied area, idealized fracture pattern models have been presented in this paper. Structural Styles and History of the Area Identifying structural styles and their histories could be one of the most important tasks in studying tectonic related fractures. These styles are generally differentiated on the basis of the extent of the basement involvement and/or the detachment of sedimentary cover. In the Khuzistan oil fields belt of southwest Iran, reservoir structures have become further complicated by the superimposition of fundamentally different tectonic regimes, which are still active based on a high concentration of earthquake foci in the area (Figure 1). Some evidence of recent activity of the regional basement controlled faults is indicated by a high concentration of the earthquake epi-centres (foci), the existence of sinuosity (bending) in the reservoir structures and high length to width ratio in most of the anticlinal structures of the Plio-Pleistocene age. The major deformation took place in the Miocene, following the collision of the Arabian plate with the central Iranian block. This convergence, with its accompanied subduction, caused the folding of sedimentary cover into the present styles; huge anticlines, with general trends of northwest-southeast perpendicular to orogenic (folding) compressional forces which are southwestnortheast trending. These styles are mostly formed during three different stages:Early orogenic shocks, which caused the early folding phase in the Miocene.Main orogenic shocks, which caused the late folding phase in the Pliocene.Recent orogenic shocks, which caused the recent folding phase. Each folding phase created its own related fracture pattern, which are discussed in this paper.