Interpretation of Aeolian Dune Geometry and Identification of Potential Barriers Based upon Image Log Interpretation

Gerard, Jean ¹; Ferrario, Luca ²; Sharky, Khaked ^3
1 Discplinas Geologicas, Repsol Exploracion SA, Madrid, Spain.
² Exploration, Eni, San Donato Milanesa, Italy.
³ Exploration, Enirepsa Gas Limited, Al-Khobar, Saudi Arabia.

Glacial conditions prevailed over Arabian plate particulartly during latest Carboniferous (Stephanian) to Earliest Permian (Asselian) times as a result of the southerly location of Gondwanaland. Thick periglacial sandstones accumulated in Haradh Fm which rest on the Hercynian Unconformity. The onset of deglaciation is recorded by a regional flooding event (Jawb Fm) likely time equivalent of Haushi Limestones in Oman (Late Sakmarian). Semi-arid floodplain red deposits and aeolian sands of the underlying Unayzah Fm accumulated under desert conditions in Artinskian and Kungunrian times followed by the onset of the regional late Permian transgression (Ash-Shiqqah Fm and Khuff Fm) over the entire area.

Dryland sedimentation was the main component of Unayzah Fm. Aeolian dunes from Unayzah Fm have a fair reservoir potential in the area as a result of the absence of clay. The understanding the bedding patterns and therefore the levels of heterogeneity supports a better prediction of the reservoir behaviour. In the absence of core data, the identification of the architecture was supported by a detailed interpretation of the image logs closely linked to conventional wireline log responses. The interpretation resulted in a sedimentological log aiming at distinguishing:

Thick intervals of stacked highly dipping foresets pointing to the same direction were interpreted as transverse dunes.

Fields of stacked smaller barchanoid dunes consisting of sandbodies with more gently dipping foresets pointing to various directions and low dip value sandy to silty bottomsets. Rather flat argillaceous intervals interpreted as wet interdunes. Semi-arid floodplain deposits dominated by rather argillaceous intervals with interstratified dipping intervals interpreted as isolated sandunes. The identification of the architectural elements is supported by outcrop analogs. In addition, identification of potential flow barriers was deduced from the sedimentological log.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009