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ABSTRACT

Texaco Exploration drilled the Mayura #1 well in the southern Pattani Basin of the Gulf of Thailand in the first half of 1994 with partners Fina and Ampolex. The well was drilled on the ramp side of a half-graben and had numerous oil shows in proximal, nonmarine reservoirs within the synrift section. One DST zone flowed oil but the permeability of the reservoir units was compromised by kaolinite in the pore system. High frequency, nonmarine sequence stratigraphy was applied to the wireline log data. Interpretation criteria are outlined suitable for application to proximal stratigraphic positions within alluvial fan to braided plain environments. A typical sequence is composed of stacked, fining-upward parasequence units that aggrade during times of relative base level rise. Each sequence shows the depositional response to factors that reorganize the drainage patterns and/or the delivery system of sediments to the basin. This response records an overall maturing-upward of depositional processes in each sequence through geologic time. Alluvial fan lithologies accumulated near the base of the sequence and braided plain to high energy fluvial sands aggraded in the middle to the top of each sequence. The resulting stratigraphic architecture formed two reservoir plumbing systems that affected the DST results in the Mayura well.

Interpretation of seismic data shows that two large-scale movements on the border fault also affected the sediments in this basin. Tectonically-controlled sediment packages formed with rapid deposition into the accommodation created by slip on the border fault. Poorly organized depositional processes dominated at this time. As the accommodation filled, the fluvial plain broadened and a more mature, fluvial plain had time to develop and aggrade. Tectonic control on the sediments reduced and the influence of other factors, like base level, increased causing a stacked sequence architecture. Each rapid slip on the border fault created a low-order, shallowing-upward sediment package. It is also interpreted to be a ‘maturing-upward’ cycle of depositional processes. The low-order tectonic and higher-order sequence stratigraphy in graben settings show a hierarchy of depositional packages that can lead explorationists to greater predictability of reservoirs within nonmarine environments.

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