Characterization of Thin-bedded Turbidite from Fine-Grained Submarine Fan Deposits
Thin-bedded deposits (< 1cm to 30 cm) comprise the "low-contrast, low resistivity" reservoirs in the Gulf of Mexico, North Sea and Nigeria. Similar deposits of the Tanqua Karoo Subbasin in South Africa are associated with the major deposition facies in the five exposed fans. In the downfan direction these facies are upper-fan nested channel complex, upper-fan overbank, mid-fan channel-levee/overbank complex, mid fan channel abandonment, and outer fan layered sheet. The facies are very fine grained ranging from shale-siltstone in the distal overbank deposits to very-fine/fine-grain sandstones in the layered sheet deposits.
Characteristics of the thin-bedded deposits from the various depositional subenvironments in South Africa are classified using, grain-size (petrography and SEM techniques), grain-orientation (anisotropy of isothermal remnant magnetization, IRMA), bed-thickness, sedimentary structures and gamma-ray measurements (field observations and profiles). The physical characteristics of thin-beds from distal overbank and channel abandonment facies are similar. They have similar bed-thickness range (2-8 cm) and sedimentary structures in the form of parallel laminations and incipient ripples. Thin- bedded deposits in the levee/proximal overbank and layered sheets have the same bed- thickness range (10-42cm) but sedimentary structures are grossly different. The layered sheets represent top-truncated Bouma Sequences while the levee/proximal overbank deposits have base truncated Bouma sequences, with an overwhelming abundance of climbing ripples. The thin-bedded deposits from the levee/overbank and layered sheet sandstones are likely to display good well performance and the other two categories not. The levee/proximal overbank deposits are additionally favored in this respect due to the possibility of these beds being connected with channel-fills. Examples of this style of enhanced bed connectivity has been observed in South Africa and from the Brushy Canyon Formation in the Delaware Basin. Overbank deposits in the Brushy Canyon also characteristically thin-bedded with well developed ripples and climbing ripples.