Tectonic and Eustatic Control on Mishrif Regional Reservoir Distribution

Abstract

Abstract This article exclusively relies on data from published literature. The development of a foreland basin in the Mesopotamian Basin of South Iraq during the deposition of the Mishrif resulted in facies stacking patterns reflecting the development of a flexural bulge. Paleogeographic reconstructions rely on carbonate facies as depth indicators, whereby rudist biostromes and coarser bioclastic debris define Shoal Complexes that separate restricted lagoonal deposits from open marine sediments. Ahmadi-Rumaila-Lower Mishrif sediments deposited into a N-S trending basin also showing evidence of precursory foreland basin tectonics: the increasing thickness of Lower Sequence deposits to E and NE reflects an increase of accommodation space in these areas. A disconformity separates the lower and upper Mishrif sequences, and represents the initiation of the flexural forebulge. The lower sequence (Mishrif mC, mB2) is characterised by a N-S oriented platform margin Shoal Complex, while during the upper sequence (Mishrif mB1, mA), a NW-SE orientation prevailed. The origin of the regionally deeper facies at the start of the Upper sequence reflects eustatic sea level rise, after which foreland basin tectonics significantly changed the paleogeography of the basin and enabled the rudist Shoal Complex to spread over a wider area. The Mishrif caprock at the top of the Upper Sequence represents a major regional exposure. Alpine 1 tectonic activity, previously associated with the Khasib, Tanuma, Sadi and Hartha Formations in the region, actually initiated earlier, impacting the upper and to a lesser extent lower Mishrif sequences. The activity is interpreted in a foreland basin setting, with structural styles similar to but smaller scale than that seen later in the Alpine 2 Zagros emplacement. Local structuration effects along pre-existing lineaments likely reactivated in different orientations due to the rotating stress fields. NW - directed push leading into Alpine 1 on N-S oriented structures may have resulted in transpressional strike slip related uplift. Such local uplift cannot however explain the observed facies stacking patterns, particularly in the Upper Mishrif.