SEQUENCE STRATIGRAPHIC ANALYSIS OF THE MID-CRETACEOUS MISHRIF FORMATION, SOUTHERN MESOPOTAMIAN BASIN, IRAQ

摘要

The middle Cenomanian-early Turonian Mishrif Formation, a major carbonate reservoir unit in southern Iraq, was studied using cuttings and core samples and wireline logs (gamma-ray, density and sonic) from 66 wells at 15 oilfields. Depositional fades ranging from deep marine to tidal flat were recorded. Microfacies interpretations together with wireline log interpretations show that the formation is composed of transgressive and regressive hemicycles.The regressive hemicycles are interpreted to indicate the progradation ofrudist lithosomes (highstand systems tract deposits) towards distal basinal locations such as the Kumait, Luhais and Abu Amood oilfield areas. Transgressive hemicycles (transgressive systems tract deposits) represent flooding of the shallow carbonate platform and are recorded in oilfields such as Amara, Halfaya and Zubair. A sequence stratigraphic framework has been constructed for the Mishrif Formation based on correlation of the transgressive and regressive hemicycles which are separated by maximum flooding surfaces. Three third-order sequences are identified which show lateral and vertical fades variations depending on relative sea-level changes. Sequence boundaries are characterized by karstic, exposure and drowning features. Middle Cenomanian-Turonian eustatic sea-level changes together with regional-scale tectonic deformation of the Arabian Plate controlled the availability of accommodation space and therefore the depositional profile during development of each sequence. Both of these factors controlled the maximum flooding surfaces and sequence boundaries which have been identified. The sequence stratigraphic key surfaces presented in this study represent typical candidate horizons or datum surfaces for future seismic or resevoir modelling studies. Also, lateral fades variations in each transgressive-regressive sequence and associated carbonate bodies (i.e. prograding shelf margin, forced regressive wedge) may form important stratigraphic traps in the Mesopotamian Basin.