Mid-Turonian–early Campanian sequence stratigraphy of northeast Iraq

Abstract

ABSTRACT A sequence stratigraphic and tectonic model for the mid-Turonian-early Campanian carbonate sequence in North Iraq was constructed based on subsurface lithologic, thin-section and well-log analyses of ten boreholes. The studied sequence is represented by the Gulneri, Kometan and Mushorah formations in the eastern sector (seven wells) and their correlative Khasib, Tanuma, Sa’adi and lower part of the Hartha formations in the western sector of the study area (three wells). Three second-order sequences (from oldest to youngest A, B and C) were identified. Sequence A consists of two third-order sequences (A1 and A2) and is represented by the mid-Turonian Gulneri Formation. Sequence B consists of three third-order sequences: B1 corresponds to the late Turonian-early Coniacian Khasib Formation and correlative lower part of Kometan Formation. Sequences B2 and B3 are exclusive to the late Coniacian-early Santonian Tanuma Formation and correlative middle part of the Kometan Formation. Sequence C is comprised of three third-order sequences of which C1 and C2 encompass the late Santonian Sa’adi Formation and correlative upper part of the Kometan Formation, while C3 corresponds to the early Campanian Mushorah Formation and correlative lower part of the Hartha Formation. Each of the third-order sequences is comprised of transgressive and highstand systems tracts, while the lowstand systems tract is restricted to sequences B2, B3, C1 and C2. These systems tracts are, in turn, comprised of vertically stacked parasequences of shallowing upward packages. The tectono-depositional model that was deduced from the sequence analysis is characterised by two flat-topped ramps that formed as a consequence of extensional tectonism. Three NW-trending normal faults are interpreted to separate the ramps whose inclined apex was continuously being eroded. The eroded detritus from the apex consists of shallow-marine carbonates that were deposited in basinal sites. Within this structural framework, the ten boreholes are placed within their appropriate depositional sites within the ramp system. Facies analysis demonstrated an array of facies associations, which generally accumulate in basins that are fed by an influx of eroded shallow-water derivatives. The depositional architecture developed in an open-marine, low-energy, middle- to outer-ramp setting in the case of Gulneri and Kometan formations. A middle–inner ramp setting is attributed to the Khasib, Tanuma and the lower part of the Hartha formations. A transition to a slope setting is suggested during the deposition of the Mushorah Formation; whereas slope-apron facies are common to all formations in the well Quwair-2. Diagenetic modification overprinted the carbonates and its effect on reservoir properties is considered. Recognition of third-order cycles demonstrates the utility of using genetic units and sequence stratigraphy to discern the depositional architecture of these formations. Local tectonic influences, which control sedimentation patterns, eustasy and sedimentation rates are additional determinants for the final stratigraphic framework.