Abstract

Seismostratigraphy, areal distribution, and depositional environment of the Early Cretaceous age Lower Thamama/Habshan carbonate depositional sequences in the SE part of Onshore Abu Dhabi were studied using seismic, VSP, and well log data. Sequence boundaries were determined from the seismic data by extracting wavelets and establishing accurate correlations between the wells and the seismic data. The Global Eustatic sea-level curve and seismostratigraphic concepts were utilized to define four depositional sequences in Lower Thamama/Habshan, and predict their environment of deposition and lithology.

Introduction

The area of the study is located in the South Eastern part of Abu Dhabi (Fig. 1). The Habshan Formation (Fig. 2), the Lower Thamama carbonates of Early Cretaceous age (Valanginian/Berriasian), exists throughout Abu Dhabi and displays varying internal reflection geometries in the seismic data mirroring variations in the environment of deposition. Changes in the internal reflection character are especially pronounced in the area of the study where it has been possible to interpret the depositional sequences, the environment of deposition, and lithology from high quality seismic data and well logs by the application of seismostratigraphic concepts as developed and applied by Mitchum, et al. (1977), Vail, et al. (1977).

During the late Jurassic, rifting and subsidence together with the sea level rise created a deep marine shelf basin (Rayda Basin) located in the northern part of present day onshore Oman (Scott, et al. 1988). Throughout the Early Cretaceous the deep basin in Oman was rimmed to the south by Habshan shallow marine, shallowing upward shelfal carbonate facies which prograded into the basin, and are considered to be the time equivalent of the deeper marine slope and pelagic carbonate facies, Salil and Rayda (Hughes Clarke 1988; Haan, et al. 1990). The deeper marine facies (Salil and Rayda) are present in east Abu Dhabi as the deep basin in Oman extended westwards into Abu Dhabi (Hassan, et al. 1975), and formed the eastern margin of the prograding Habshan shallow marine carbonates. In this study we present evidence from the seismic and the well data that the Habshan Shelf margin in north Oman as defined by Haan, et al., swings into Abu Dhabi along a NW trend, and the Lower Thamama/Habshan were deposited as four distinct sequences in SE Abu Dhabi in response to the changes in the sea level and the sediment supply.

Database

The study area is covered by approximately 3500 km of various seismic data, mostly high multiplicity *60 or 120 fold, 25 m group interval) and good quality. The seismic grid spacing is approximately 2km by 2km, allowing horizon correlations at the wells to be extrapolated with confidence over the study area.

There are seven wells which penetrated the Habshan Formation in the study area. Most of these wells have VSP surveys (Walkaway, zero-offset or both), allowing the generation of calibrated synthetic seismograms and VSP corridor stacks for a proper correlation of the surface seismic with the wells (Fig. 3). When the amplitude and phase match of the seismic data with the well synthetics was less than desirable, the seismic data were deterministically deconvolved using the extracted wavelets at the well ties to improve the data quality.

Habshan Seismic Sequences

There is little impedance contrast at the Lower Cretaceous-Top Jurassic boundary since the Upper Jurassic Hith Anhydrite is absent in the area. Also some multiples generated in the Upper Cretaceous often interfere with the weak primary reflections generated at the Top Jurassic boundary. Therefore the Base of Habshan cannot be always interpreted directly from the seismic data. Instead, a regionally continuous seismic event within the Jurassic (near Top Jurassic) was interpreted, where possible, as a guide to Base Habshan throughout the area (Fig. 3).

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