Regional Characterization and Stochastic Object Modeling of the Pre-Khuff Clastics in Abu Dhabi

Abstract

According to the drilling results in Abu Dhabi, the sweet-gas prospective of the Silurian-Permian pre-Khuff clastics is risked by the reservoir uncertainty controlled by different types of clastics. Previous studies focused on well-scale deposition analysis barely reflects the regional sedimentary geometry nor predicts the distribution of different sand-bodies in the pre-Khuff sequences. To better map the play-fairway and to mitigate the drilling uncertainty, it is of significant importance to model the regional depositional settings with different reservoir sands based on the integrated sediment study. The regional sequence framework was based on the biostratigraphy study and log cycles. To model the original deposition, eroded sequences and re-worked sediments were re-constructed based on the near offset data. The plate-scale depositional model was integrated with Abu Dhabi sedimentary records based on cores, cutting description and log interpretation to create a statistics database of depositional settings in each sequence. Training images that accommodate the changes in sedimentary records were created adapting seismic patterns and subsequently were used for the multi-point statistics and object modeling. After the Silurian marine-delta deposition, the continuous non-deposition in southern onshore area and the intermittent erosion in offshore due to periodic salt movement suggests a successive deepening of the paleo-relief to northern Abu Dhabi. The sea-level drop since Silurian creates the marine to continent transition and results in the prograding of continent sediments to the north following the relief of the Paleozoic salt withdraw basin. The Devonian dolomite-anhydrite sediments indicate the extension of marine carbonate from Saudi, Iraq and Kuwait to northwest offshore Abu Dhabi. After the Carboniferous glaciation, the terrigenous sediments become dominant in Berwath, Unayzah and Basal Khuff Clastics. The south and east of Abu Dhabi is pre-dominantly fluvial original, while the west area records both fluvial and Aeolian deposition. The widespread of Aeolian sands is compromised by the flooding events of fluvial and sheet-flow. The mud coated fluvial sands can preserve fairly better permeability than the well-sorted Aeolian sands due to the inhibition to quartz cements, which makes it also primary reservoir target. The Western and central offshore areas at the transition of Aeolian-fluvial sands and floodplain muds are more favorable for the development of reservoir and seal interlayering. Rather than assigning one dominant facies to each pre-Khuff formation as the previous work, the regional stochastic model differentiates fluvial channels, crevasse splays, aeolian dunes, sheet-flood and marginal marine/lacustrine deposits in each sequence based on solid well data and regional statistics. Therefore, improve the understanding of the stacking patterns of reservoirs and seals that can be used to characterize the pre-Khuff hydrocarbon behavior, integrating studies of structure growth, burial and maturation history.