Online Journal for E&P Geoscientists

Wharton, Stanley R.^*1; Bakhiet, AbdelFattah ¹; Kumpas, Michael ²; Lawrence, Paul ¹; Tang, David ¹; Gregory, Arthur ¹
(1) Saudi Aramco, Dhahran, Saudi Arabia. (2) Consultant, Marlow, United Kingdom.

The Jurassic period worldwide is defined by regional sea-level changes punctuated by major transgressions and regressions. These events have created major stratigraphic sequences throughout the world and resulted in the deposition of key reservoirs, source rocks, and seals throughout this stratigraphic interval. On the Arabian Platform, the Jurassic sequences are prolific and contain significant oil and gas accumulations. Additional hydrocarbon potential may exist based on a more detailed understanding of the stratigraphic gaps/hiatus events, and their impact on the redistribution of reservoir rocks, the maturation of the source rocks and the hydrocarbon migration.

In the past, significant work, based on core and outcrop data, has been done on dating the Jurassic ammonite biofacies to help identify the key hiatus events, but less effort has been made on the use of the seismic record to assess basin history and evolution. This study was conducted using 2D and 3D seismic chronostratigraphy models and Wheeler transforms to assess the depositional records between the Kimmeridgian (Arab) and the Carnian (Jilh) stages across the Arabian platform. Chronostratigraphic models for several areas in the basin were constructed from a dense array of seismically generated horizons, targeting each sample in each seismic trace. These models enabled the key subseismic truncations in the data to be revealed, and were then calibrated with existing outcrop and well observations. Use of Wheeler transforms, generated from flattened horizons, allowed a dynamic assessment of the basin history and the mapping of key events, including: transgressions, regressions and hiatus signatures.

Several of the key gaps/hiatus events defined from biostratigraphic outcrop assessments can be correlated with interpreted hiatus events in the seismic record. The key hiatus events identified in the Wheeler assessment includes at least two major regressions in the study area. A significant event is in the Bathonian (Mid Bathonian to Mid Callovian, based on outcrop study) within the Atash Member. This regression lasted 4-6 Ma, based on work in the Sinai and elsewhere, and may be more significant than initially considered for Arabian Jurassic plays.