UNRAVELLING THE COMPLEXITY OF THIN (SUB-SEISMIC) HETEROGENEOUS CARBONATE RESERVOIRS: AN INTEGRATED STUDY OF THE ALBIAN MAUDDUD FORMATION IN THE GREATER BURGAN AREA, KUWAIT

Abstract

The Albian Mauddud Formation is a prolific reservoir in Kuwait and nearby countries such as Iraq and Iran but has received far less attention than the under- and overlying units (the Aptian Shu'aiba and Cenomanian Mishrif Formations). Detailed reservoir characterization studies of the formation are required to support field development and improved / enhanced oil recovery (EOR) programmes. In this study, 26 wells penetrating the Mauddud Formation within the Greater Burgan area of Kuwait (Burgan and neighbouring fields) were investigated, integrating the logging of 910 ft of core with petrographic investigations of 113 stained and impregnated thin sections. In the Greater Burgan area, the Mauddud Formation can be divided into a lower Clastic Member and an upper Carbonate Member which is the main focus of this paper. The primary objective of the study was to present a new characterization of this thin, heterogeneous carbonate reservoir by integrating facies analysis and sequence stratigraphy with a detailed petrographic investigation. A second objective was to identify the relative importance of depositional characteristics and diagenesis on the distribution of reservoir properties.

Sandstones in the Clastic Member of the Mauddud Formation were deposited on a delta which passed laterally to the north and east into a carbonate platform. During subsequent regional flooding, increased carbonate production resulted in the development of a larger-scale carbonate platform covering the entire study area. The Burgan field area was part of the proximal zone of this carbonate platform. A number of depositional environments were identified by integrating core and thin section data. These range from outer platform to mid- and inner platform, the latter including both high- and low-energy settings (shoal, shoreline; and lagoonal respectively). Mud-supported textures characteristic of low-energy inner-platform and mid- to outer-platform settings are volumetrically dominant in the Mauddud Carbonate Member.

Sequence stratigraphic analysis suggests that the Mauddud Carbonate Member is part of a major regressive phase (or highstand systems tract) of a third-order sequence, with the regional-scale K110 MFS positioned close to the transition with the underlying Clastic Member. Two 4^th order transgressive – regressive (TR) cycles or sequences, M1 and M2, were identified within the Carbonate Member. The top-Mauddud surface corresponds to a sequence boundary with long-lasting subaerial exposure during the latest Albian and is characterized by both micro- and macroscopic karst features (calcite dissolution vugs and recrystallization in thin sections; and cavities in cores). This study demonstrates that the Burgan field area experienced significant uplift, with increased differential erosion and/or non-deposition of the upper M2 TR cycle towards the southwest.

Eogenetic marine and meteoric calcite cements partially fill macropores close to the tops of the TR cycles and remaining macropores were occluded by shallow-burial calcite cements. Cementation by ferroan dolomite cement, which resulted from the increased overburden and associated chemical compaction, has almost entirely occluded residual macropores in the lower part of the Mauddud Carbonate Member, close to the underlying siliciclastic deposits of the Clastic Member. As a consequence, porosity is mostly preserved in the middle part of the Carbonate Member. The predominance of mud-supported textures in these intervals together with the effects of diagenesis explain the widespread occurrence of microporosity within both micritic matrix and grains. In the Greater Burgan area, meteoric diagenesis associated with subaerial exposure(s) of the top-Mauddud preserved and/or enhanced micrite microporosity. Plug porosity may reach 35% or more, but permeability does not exceed 100 mD.

The Mauddud Carbonate Member in the Greater Burgan area is a thin, microporous reservoir, with a dual porosity component which is linked to karst alteration and not to tectonic fracturing. The distribution of reservoir properties results from a combination of primary depositional characteristics and a multiphase diagenetic overprint. This study will guide the future development of the Mauddud reservoir in the Greater Burgan area. It also demonstrates the importance of an integrated approach to constrain sub-seismic heterogeneities in carbonate reservoirs.