Genesis and Distributions of Typical Architecture Sand-Bodies and their Evolution in Relative Sea-Level Fluctuations for a Giant Cretaceous Delta of the Upper Shale Member, Zubair Formation in the Rm Oilfield

Abstract The Upper Shale (US) sandstone is one of the most important reservoirs in the Rm oilfield, south Iraq. It belongs to the upper part of the Zubair Formation in Early Cretaceous and deposited in giant delta environments. Although US multiple sand-bodies are superimposed and distributed vastly, its internal configurations and distributions in different intervals vary significantly, and therefore, it is very important to determine the distribution and architectures of different sand-bodies for the successful development in US. The criterion of the US lithology identification is constructed by core calibration and wireline-logging response analysis. Different architectures in genetic units and their associations are revealed after integrating core sedimentary sequences, wireline-log interpretation and genetic analysis of sandbodies in stratigraphy correlation sections. By means of restoration of ancient landform in sections, the relationship between relative sea-level fluctuation, paleogeomorphology and sandbody development in different depositional-facies tracts are studied for disclosing controls and evolutions of different sandbody architectures. Furthermore, the sandbody connectivity classification is applied by architectures, lithology and petrophysical properties, and finally, distributions of different connectivity sandbodies are predicted based on genetic correlation in development well patterns. Results indicate that sandbody architectures in genetic units can be identified as main channel in axis (MCHA), distributary channel/channel margin (DCH/CHM), main channel in axis with tide-wave influences (MCHA-TWI), mouth bar (MB), channel abandonment (CHAB), sandsheet/sandflat (SSht/SFlt) and distal distributary channel (DDCH). These genetic units are usually distributed in 6 architecture associations, such as TypeA1, TypeA2, TypeB1, TypeB2, TypeC and TypeD, which represents associations of MCHA, MCHA-TWI, DCH-MB-DCH, DCH-SSht-DCH, DDCH-SSht-DDCH and SFlt-CHAB-SFlt respectively. On the sequence boundary and downfold, TypeA1 association is well developed, and on the maximum-flooding surface, TypeA2 is developed if in strong river-tide-wave actions. The sandbody architecture will change from MCHA, DCH-MB-DCH and DDCH-SSht-DDCH from the root to remote part of delta-front as relative river-marine actions change. The sandbody connectivity is classified into 4 types based on architectures, lithology and petrophysical properties, in which, the best TypeA, including TypeA1 and TypeA2 associations, is clean fine-grained sandstones with 6-24m thickness and 200-15000m connection in area; TypeB, including TypeB1 and TypeB2 associations, has 2-8m thickness and the second level connectivity; TypeC and TypeD, representing synonym architecture associations of thin, heavy shale content sandstones, are very poor in permeability and connectivity. In fact, as proved by dynamic data, the variations of formation pressure coincide with the distribution of sand bodies in different architectures and connectivity. The methodology and results in this paper are of great references for US and similar reservoir characterization and development