Sequence Stratigraphic Framework of the Arab Formation Reservoirs, Dukhan Field, Qatar

Abstract

Abstract In the Upper Jurassic Arab C and D reservoirs, three 3rd-order sequences are interpreted in Dukhan Field, Qatar and are refered to as the Kim1, Kim2, and Kim3 sequences (after Hardenbol et al., 1998). At Dukhan, the Arab C and D reservoirs have produced since 1949. Over 800 wells have been drilled in Dukhan, 180 of which have been cored. Based on sedimentological analysis of the cores, environments of deposition (EODs) have been assigned to cored wells. Twenty three EODs were identified and grouped into five facies tracts: Sabkha, Beach, Ramp Interior, Ramp Crest, and Outer Mid Ramp. Parasequences and parasequence sets were recognized in the core data, and sequences were interpreted from the parasequence stacking patterns as well as evidence for exposure. Log-based correlation provides a method for extending the sequence stratigraphic framework from cored to uncored wells across the field. Within a sequence stratigraphic framework, the spatial positions of facies tracts and corresponding EODs are predictable. Based on lateral changes in EODs and onlap patterns, we have determined that deposition dip is towards the northeast. Capping the Kim1 sequence, the Arab D consists of a 20 foot thick interval of cross-bedded, coated-grain grainstone in the southern portion of the field. To the north, the grainstone package of the Kim1 sequence passes into a deeper-water EOD with a higher percentage of packstone and wackestone. Above the Kim2 sequence boundary, a transgressive wedge thins toward the south. The Kim2 sequence contains Ramp Interior and Beach facies tracts that prograde to the northeast. The top of this sequence occurs in the Sabkha facies tract. The Kim3 sequence is initiated by the Lower Qatar Anhydrite, a regional feature that forms a seal between the Arab D and C reservoirs. The transgressive system tract of the Kim3 sequence is capped by coarse grain beaches and shallow subtidal lagoon EODs of the lower Arab C reservoir. The maximum flood is indicated by distinctive thrombolitic horizons in core. The upper Arab C represents the highstand of the Kim3 sequence and is composed of upward thinning and increasingly restricted EODs of the Sabkha facies tract. In the Arab C and D, reservoir quality is highly influenced by depositional texture. Well-sorted, cross-bedded grainstones in the shoal and beach EODs consistently have excellent interparticle porosity with permeability over 100 mD and commonly exceeding 1000 mD. Deeper-water EODs consists of a greater percentage of packstone and wackestone with intraparticle porosity and microporosity. These low-energy EODs typically exhibit permeability less than 20 mD. Sabkha EODs are of low to moderate reservoir quality except where dolomitization has improved permeability. Placing EODs in a sequence stratigraphic framework provides a method by which we can predict lateral changes in rock properties and, within 3D geologic models, distribute rock properties insightfully.