Exploration Play Analysis from a Sequence Stratigraphic Perspective, by John W. Snedden, J. F. (Rick) Sarg, and Yudong (Don) Ying, #40080 (2003).

²ExxonMobil Upstream Research Company, P.O. Box 2189 · Houston, TX. 77252-2189 (don.ying@exxonmobil.com).

Examination of exploration drilling histories for many different global basins indicates a counter-intuitive temporal and spatial pattern in the way hydrocarbons are sometimes discovered. Conventional wisdom holds that for any given basin or play, a plot of cumulative discovered hydrocarbon volumes versus time or number of wells drilled usually show a steep curve (rapidly increasing volumes) early in the play history and a later plateau or terrace (slowly increasing volumes). Such a plot is called a creaming curve, as early success in a play is thought to inevitably give way to later failure as the play or basin is drilled-up. It is commonly thought that the "cream of the crop" of any play or basin is found early in the drilling history.

By examining plays or basins with sufficiently long drilling histories and range of reservoir paleoenvironment and trap types, one actually finds two or three "terraces" to the creaming curve. The first string of successes in a given basin usually corresponds to exploitation of the highstand systems tract or sequence set reservoirs developed in updip structural traps. These reservoirs are typically marginal to shallow marine "shelfal" deposits, laterally continuous but lacking internal sealing facies and are seldom self-sourcing. The second or third terrace in the creaming curve usually involves the lowstand reservoir component (systems tract or sequence set), which is often developed in downdip deepwater or slope paleoenvironments. Transgressive (systems tract or sequence set) reservoirs, typically shallow marine shelfal sandstones that are sometimes self-sourced, are variably developed and may or may not occupy the second terrace of the creaming curve. These trends hold true for both 2^nd-order (3-10 my) and/or third-order (1-3 my) stratigraphic cycles, depending upon the scale of the basin or play.

This analysis fits well with the definition of an exploration play provided by Magoon and Sanchez (1995): a fully developed play is the simple volume difference between the petroleum system capability and the current discovered hydrocarbon volumes (commercial or not). Where the difference is large, either the petroleum system has significant leakage problems (e.g., Barents Sea Mesozoic play) or the lowstand systems tract or sequence set has not been fully exploited.

Examples supporting these ideas are drawn from several global basins (Gulf of Mexico Miocene, Norway Upper Jurassic, Mahakam Delta, Texas Wilcox). Case studies demonstrate how critical elements of exploration risk shift from trap and seal in highstand plays to reservoir and source in lowstand components of these plays.

Abstract