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Giant Gas Fields Offshore Tanzania, Braided Deep Water System Within Extremely Sandy Turbidite Systems Presented by: A. Ponten

Abstract

Mid Cretaceous turbidite strata of the Saffron Formation constitute the reservoir of giant gas fields in block 2 offshore Tanzania. Excellent quality core makes it possible to calibrate logs and seismic with sedimentological information. The reservoir is a very sandy deep water system with Net/Gross estimates of 80-85%. The reservoir interval is defined by a basal unconformity, probably representing a sequence boundary. Abrupt changes in vertical facies associations combined with a major basinward shift in facies suggest higher order sequence boundary control on parts of the reservoir sands. Even with good estimates of NtG there is significant uncertainty related to the internal architecture and facies-distribution in the reservoirs. The sandiness of the system and scours and disconformities at all scales together with partly contradicting seismic observations of channel and lobe features has stimulated us to propose a depositional model with rapidly shifting channel systems where both vertical and lateral amalgamation contributed to the final geometry. This model deviates from traditional deep water fan and lobe models and also differs from common turbidite channel and channel complex models. The sand is overall well sorted and in the range of fine to medium grain sizes, even though finer grained intervals and coarser grained lag deposits occur. Only one good candidate for draping shale within the reservoir interval is identified in the core data. Sandstone beds are 0,5 – 1 m thick and show only thin fine grained bed caps – if at all. A large proportion of the sandstone beds in the reservoir are totally dominated by water escape structures. The vertical stacking and rapid deposition of sediments inferred from the water escape structures points to a long lasting, structurally controlled, point of sediment entry. The vertical stacking of strata also suggests deposition in a loosely confined slope setting that allowed for thick accumulation of sand-rich sediments on the slope for a long period of time. This situation created a semi-permanent transition zone from well-defined channels to more unconfined deposits at the entry point to the loosely confined basin, which also explains the sandiness as well as the overall lack of levee and overbank facies. A depositional model involving a braided deep water fan adequately explains the sedimentary facies in cores and also bridges the apparent contradictions between observations made in core and on seismic.