Facies and Lithofacies of a Mid-Cretaceous Sarvak Carbonate Reservoir in SW Iran: A Case Study
Ali Asghar Taghavi1 and Atle Mørk1, 2
1Department of Geology and Mineral Resources Engineering,
NTNU, Norwegian University of Science and Technology, S. Sælands v 1 N-7491, Trondheim, Norway
2SINTEF Petroleum Research, S. P. Andersens 15B, N-7465, Trondheim, Norway
The middle Cretaceous shallow shelf carbonate ramp, the Sarvak Formation, is among the best producing reservoirs units in Southwest Iran.
Based on core description and thin section studies eight facies association, formed by fourteen lithofacies, are defined. Energy level, fossil content, texture and structure are the main parameters to define the facies. The facies represents: lagoon, patch reef, shoal (barrier and local), reef, fore reef, shallow open marine, outer shelf and intrashelf basin.
The fourteen lithofacies are defined based on texture and content. Wackestone and packstone with prevailing benthic foraminifera have been deposited in lagoonal environment. Grainstones are composed of rudists, peloids and bioclastic allochems and indicate high energy depositional environment that can be formed in lagoonal setting, as barriers and also seaward of the reefs. Rudistid floatstones are interpreted as derived from local patch reefs where the rudists were abraded in lagoonal environments. They may form in fore-reef setting near reef structures. Rudistid wackestones/packstones were deposited in the fore reef environment. Peloidal and bioclastic wackestones/packstones were deposited in shallow open marine environments. Mixed pelagic-bioclastic wackestone has been deposited on the outer shelf, and pelagic deposits are interpreted as intrashelf basin deposits. Dolomite is classified as a separate lithofacies.
Three main sequences have been recognized: US1, US2 and US3, from base to top. They form the upper part of the middle Cretaceous second order sequence of Sharland et al. (2001, GeoArabica). US1 grades from intrashelf basin deposits in the lower part to shallow shelf in the upper part. This sequence is characterized by a thin interval of transgressive deposits and a thick regressive interval. Intrashelf deposits have been deposited in a transgressive and early highstand system tract in the lower part that changes to shallow shelf deposits in the upper part. The maximum flooding surface is located within high gamma bearing pelagic foraminifera wackestone. US2 consists of a transgressive-regressive succession. It started by lagoonal deposits and change to alternative shallow open marine and high energy intervals in the upper part. The maximum flooding surface is located within the outer shelf deposits. US3 characterizes a thick regressive system which is overlain by a late Turonian unconformity. It begins by deposition of shallow open marine and grades to shoal and patch reef deposits and is overlain by prograding lagoonal deposits.
Porous intervals exist in the upper part of sequence US1 in leached fore reef rudistid packstone and dolomite intervals, in sequence US2 both in peloidal/rudistid and bioclatic grainstones of shoal deposits and leached wackestone/packstone successions and in the upper part of sequence US3 in highly leached lagoonal deposits and rudistid floatstone patch reef intervals.
Figure 2. Main sequences in the upper Sarvak Formation and the accommodation space indicating transgression and regression of shoreline. The porous intervals are located mainly in regressive part of sequences US1, US2 and US3 and represent both facies and diagenetic effect. The sequence US2 comprises more porous intervals.