Spatial Distribution and Connectivity of thin-bedded crevasse splay sandstones; outcrop-analogue studies for tough gas reservoir potential

A majority of the volume of alluvial sequences consists of fine-grained floodplain deposits (Kraus, 1987; Kraus & Gwinn, 1997). Traditionally, these deposits are seen as non-reservoir rock since their permeability is very low. This research shows that crevasse splay deposits that occur in these floodplain fines can comprise stacked complexes of splay channel sands and thin sheet sandstones. These crevasse splay deposits may provide additional production as secondary reservoirs in alluvial systems. Although much research on fluvial reservoir characteristics was carried out, this mainly focuses on properties of main river channel sandstones for primary production. Therefore, little is known about lateral characteristics of stacked crevasse splay deposits in floodplain fines. In order to gain more knowledge on depositional mechanisms and lateral reservoir properties of crevasse splay deposits, a fieldwork study and core description were carried out. During the fieldwork, crevasse splay features such as size, shape and grain sizes of ancient crevasse deposits were mapped. The outcropping sequence was captured in sedimentary logs. The fieldwork results were compared to two analogues: core descriptions of intervals at reservoir depth and a modern day crevasse splay. Both analogues have a similar depositional environment to the fieldwork area. With these results volume calculations were carried out on different scenarios, assuming crevasse splay deposits as a gas reservoir. This research shows that crevasse splay deposits have reservoir potential. Stacked crevasse complexes with a high (70%) net sand content were found to display typical thicknesses of 1-2,5m. These deposits are laterally extensive. This research presents a length of 1,5 km, but even larger examples are found in literature with lengths over 2 km. Grain sizes are decreasing with distance from river channel and crevasse channels. The correlation of grain size and distance is significantly better for distances to crevasse channels than for distances to the river channel. Depositional mechanisms that create lateral thickness variations create increased connectivity between crevasse sandstones. A possible further increase in connected area as a result of interfingering of different crevasse systems is described.

Subject

fluvial deposits
tough gas
crevasse splays

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Student theses

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master thesis

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