Exploration Play Analysis from A Sequence Stratigraphic Perspective

John W. Snedden and J. F. (Rick) Sarg: ExxonMobil Exploration Co., Houston, TX; and Yudong (Don) Ying, ExxonMobil Upstream Research Center, Houston, TX

Abstract

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 flattening out later (slowly increasing volumes). Such a plot is often 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 having 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 generally corresponds to exploitation of the highstand systems tract reservoirs developed in updip structural traps. The reservoirs are typically marginal to shallow marine "shelfal" reservoirs, laterally continuous but unable to develop internal facies sealing and seldom self-sourcing. The second or third terrace in the creaming curve usually involves the lowstand systems tract, downdip reservoirs which are often developed in deep water or slope paleoenvironments. Transgressive systems tract 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 2nd-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 et al. (1996): 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, significantly, the lowstand systems tract has not been fully exploited.

Examples supporting these ideas are drawn from many global basins (e.g., Texas onshore, deep water Gulf of Mexico, Mahakam Delta, Norwegian Sea, and North Sea). Case studies demonstrate how critical elements of exploration risk shift from trap and seal in highstand plays to reservoir and source in lowstand plays.