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ABSTRACT: Sequence stratigraphy and seal quality prediction in Block B, Natuna Sea: the relationship between top seal quality, lithofacies, and depositional environments

Suter, John R. and Peter D'Onfro , Conoco, Inc, Houston, TX

Top seal is a major element of exploration risk. During a regional exploration project in the West Natuna Basin, three major Miocene seal intervals were evaluated using seismic mapping, well log and whole core interpretation, and mercury intrusion analyses. The Barat Formation, a lacustrine silty clay, forms the top seal for the Udang Field. Seal capacity is generally excellent, but an abrupt coarsening upward succession at the top of the Barat warns of deteriorating seal quality within more proximal environments. Lower Arang deposits span several sequences and multiple reservoir and seal facies, including marginal marine, nearshore, and non-marine sediments. This interval comprises the main reservoir at Belida Field, which is sealed by a shale of local extent. Lithofacies and seal quality of Lower Arang samples varied widely between sequences, systems tracts, and parasequences, and within depositional environments. Seal quality is a major risk in this interval and may be a controlling factor on hydrocarbon accumulations. The Muda Formation comprises fine-grained open marine sediments with excellent seal capacity. However, several significant surfaces, identifiable in cores by changes in bioturbation style, occur within the succession. We interpret these surfaces as parasequence and sequence boundaries, which indicate the potential for significant regional variation in Muda lithofacies and seal capacity. Proper evaluation and prediction of top seal requires knowledge of seal capacity, integrity, and continuity. Lacking this information, prediction must rely on conceptual models and analogs that place potential seal rocks in depositional and sequence stratigraphic contexts.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia