Architectural Analysis of Deep-Water Sandstones: Implications for Exploration and Production

Sullivan, Morgan, Gerrick Jensen, and Frank Goulding, ExxonMobil Exploration Company, and David Jennette, Lincoln Foreman, and Anthony Sprague, ExxonMobil Upstream Research Company

Abstract

Architectural styles of sand bodies that have been deposited in deep-water settings are highly variable. This variability in sand body geometry and continuity affects both the exploration and production potential of deep-water sandstones. Commonly, however, these key architectural variations are at or below seismic resolution and well penetrations are frequently limited and thousands of feet apart. The high cost of exploration and development in the Gulf of Mexico, offshore West Africa, and offshore Br azil require accurate pre-drill prediction and post-drill assessment of deep-water reservoirs, in order to ensure an economic project. Outcrops span this critical gap in both scale and resolution between seismic and well-bore data. Integration of appropriate outcrop analog data can provide the net to gross and geometric properties required for interpreting reservoir architecture at a sub-seismic or flow-unit scale.

The Lower Permian Skoorsteenberg Formation in Tanqua Karoo Basin, South Africa and the Lower Carboniferous Ross Formation in the Clare Basin, western Ireland are both composed of stacked turbiditic sandstones and mudstones deposited within a channelized to unconfined basin-floor fan setting. Based on detailed characterization, the stacked sand-rich turbidite successions can be divided into proximal, medial and distal fan settings each with its own set of key characteristics.

The most proximal outcrops (toe of slope, proximal fan) are characterized by narrow, erosionally based, channelized sandbodies that are dominated by amalgamated massive sandstones. Toward the channel margins, however, amalgamated sandstones are typically replaced by progressively thinner bedded, less amalgamated sandstones.

The medial fan settings is dominated by compensationally stacked, very broad (high aspect ratio) channel complexes. The bases of these complexes tend to be nonerosional or only slightly erosional, suggesting that they are primarily an aggradational depositional element. These channelized sandbodies can also be further subdivided into distinct channel-axis and channel-margin associations. Channel-axis deposits are characterized by highly amalgamated massive sandstones. Away from the axis, beds become distinctly less amalgamated and extremely continuous to produce laterally extensive, layered wings at the channel margins.

The distal fan setting is comprised of extremely broad channels or what more correctly can be termed sheets. They commonly display compensational stacking due to depositional highs related to underlying channel/sheet complexes. Over all this is a very layered reservoir style dominate by non-amalgamated massive sandstones and interbedded mudstones. The Skoorsteenberg and Ross formations have a sediment compositions and architectures very similar to the many deep-water reservoirs. Therefore, net to gross, dimensional and architectural data from these outcrops can be used to help assess future prospects and newly discovered fields with similar characteristics. Furthermore, the integration of seismic, well, core, and outcrop data provides the framework to maximize the architectural controls on reservoir development and performance.