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Paralic Oil and Gas Fields - What Makes Them Distinctive: From the Pore Scale to the Reservoir Scale

Tony Reynolds, BP, Middlesex, England

Paralic depositional systems occur at or near to sea level, and comprise deltas, shoreline-shelf systems, and estuaries. Reservoirs in such successions have recovery factors that range from 5 to 70% for oil, and initial reserves that extend from single-well, low-volume pools to super giant fields containing reserves in excess of one billion barrels. The value of any paralic development, irrespective of its size, can be characterized by assessing the reservoir heterogeneity across a range of scales.

At the large scale, paralic depositional systems respond sensitively to sea level change, often by large lateral shifts in facies belts. As a result, deltas, estuaries, and shoreline-shelf systems, (and their sub-environments, which reflect the dominance of fluvial, and mixed fluvial and marine processes) are commonly interleaved forming highly layered successions. Each layer may form a discrete reservoir, pressure isolated from those above and below. At an intermediate scale the discrete layers display a wide range of sand-body types (such as channels, splays, and shoreline-shelf sands) with each sand-body type having distinct dimensions (widths, lengths, thickness, and shape) and internal properties (for example, coarsening or fining upward). Consequently, paralic reservoirs can range from thick, extensive sheets to thin, laterally restricted sandstones. Thick, extensive sandstones typically have structurally defined spill points, extensive aquifers, and good reservoir properties. Such sands have high recovery, good reservoir performance, and form primary completion targets. Thin, laterally restricted, isolated sandstones have lower recovery factors and poor performance. In such instances each isolated sand may have its own set of hydrocarbon contacts, and a distinct outline that reflects stratigraphic trapping.

In addition to depositional complexity, faulting is a recurring risk. Reservoirs tend to be less than 30 m in thickness, and are often offset by small faults that may seal due to sand-shale juxtaposition and/or shale smear. As a consequence, sealing faults are common trap elements, and causes of compartmentalization within high net:gross systems. The combined complexity of sands and faulting commonly results in one or two dominant reservoirs that are exploited first, and a tail of minor reservoirs that are exploited after the main reservoirs are in decline. The complexity also impacts drilling patterns, well selection, requirements for multizone completions, and uphole recompletion.

At a small scale, paralic reservoirs are distinctive in being dominated by laminated sandstones. The nature of these laminae (thickness, grain size, angle, shape, sorting) and how they truncate are commonly indicative of depositional environment and allow sand-body type to be determined. In addition, their impact and that of associated shales on permeability anisotropy and resulting fluid flow is increasingly well understood.

Indeed, heterogeneity in paralic reservoirs is increasingly well understood at all scales. Advances in (1) seismic data acquisition and processing, (2) application of sequence stratigraphic concepts, (3) availability of robust sedimentological models, (4) improvements in analog databases, and (5) increased integration have all played key roles in allowing heterogeneity from the pore scale to the reservoir scale to be described, and reserves to be estimated earlier in field life allowing better decisions and sooner.

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