Geological Attributes of Fine-Grained Turbidites as Unconventional Reservoirs

Stow, Dorrik ¹; Gardiner, Andy ¹
(1)IPE, Heriot Watt University, Edinburgh, United Kingdom.

Within turbidite systems, the primary focus has been on coarse-grained conventional reservoirs, and much less is known about associated fine-grained sediments. Yet these are volumetrically the dominant facies in deepwater, form important marginal reservoirs and have great potential as tight reservoirs. They are also significant as source rocks and seals. Based on a large number of studies of modern, ancient and subsurface systems, we define the principal genetic elements of fine-grained deepwater facies and synthesise their geological attributes. Discrimination of these elements has great significance for exploration and production of tight reservoirs in deepwater systems, as well as in characterization of deepwater shale properties.

The principal architectural elements include: non-channelised slope-aprons, channel-fill, channel levee and overbank, turbidite lobes, mass-transport deposits, contourite drifts, basin sheets and drapes. These comprise a variable intercalation of fine-grained facies - thin-bedded and very thin-bedded turbidites, contourites, hemipelagites and pelagites - and associated coarse-grained facies. Characteristic attributes used to discriminate between these different elements are: facies and facies associations; sand-shale ratio, sand and shale geometry and dimensions, sand connectivity; sediment texture and small-scale sedimentary structures; and small-scale vertical sequences of bed thickness.

We identify four fundamental variables, derived from attribute combinations, which influence vertical and horizontal flow: 1. The sand connectivity index, which we derive from the nature of bed/lamination cross-cutting relationships. 2. The sediment textural index, which is derived from the mean grain-size property and varies with element type, proximality, facies, fabric and diagenesis. 3. The proximality index, which is derived from Bouma/Stow sequence combination and selected facies ratios. 4. The micro-fracture index, which is derived from the micro-fracture density, style and distribution. For selected element types and architecture, we have run flow simulation models for combinations of attribute variable. The run simulations yield very significant differences in vertical and horizontal flow behaviour for different turbidite settings that we suggest are a valuable predictor for reservoir performance. The next step will be to validate these models against actual production histories in tight and very tight reservoirs.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.