Sequence Stratigraphic Framework for Prediction of Shallow Water Flow in the Greater Mars-Ursa Area, Mississippi Canyon Area, Gulf of Mexico Continental Slope

Charles D. Winker and R. Craig Shipp, Shell International E&P, Houston

Abstract

Shallow water flows occur when a well is drilled underbalanced into near-surface, overpressured sands. Shallow water flows have been one of the most problematic aspects of deep-water drilling operations in the Gulf of Mexico, causing premature abandonment of many wells and expensive delays in many others; total costs to the industry have been estimated at hundreds of million dollars. In recent years, engineering solutions have been largely successful in preventing or managing Shallow water flows; nevertheless, it is still vital to predict both the depth of occurrence and degree of overpressure in near-surface sands. Seismic signatures of these sands range from ambiguous to invisible, and pressure prediction methods from seismic move out velocities are inconsistent. Therefore, the most practical prediction methods are based on geologic and engineering data from previous wells and boreholes (>50 surface locations in southwestern Mississippi Canyon alone), including LWD logs, direct and indirect overpressure indicators, and drilling summaries. To make lateral predictions from these well data, a local sequence stratigraphic framework has been established, which utilizes overlapping conventional and high-resolution 3D seismic surveys tied to a regional framework based on 2D and 3D seismic data.

Occurrences of near-surface sands and their potential for overpressure on the Gulf of Mexico continental slope are intimately related to late Pleistocene sequence stratigraphy of the Mississippi delta and fan. This is best illustrated in the Greater Mars-Ursa area and vicinity, site of the industry's greatest SWF-related losses. The base of the main trouble zone in the Greater Mars-Ursa area is defined by a sub-regional sequence boundary (SB) (~Globorotalia flexuosa, P1), typically marked by a "soft shale" and in many minibasins (but not in Greater Mars-Ursa area)) by an onlap surface. Interpretation of the post-SB stratigraphy is complicated by numerous mass-transport complexes of diverse origin and by abundant gas-related amplitude anomalies.

Overlying the P1 SB is a basin-floor fan system, informally known as the MC Blue Unit, ponded on the slope by subtle antecedent topography. This basin-floor fan is apparently contemporaneous with the last lowstand delta complex of the Mississippi (Stage 2 and possibly 4); the equivalent unit has not been identified on the Mississippi fan. The basin-floor fan is characterized by abundant, but highly discontinuous sands, typically in vertically separate pressure compartments. On high-resolution data, the seismic character consists of both layered, sub-parallel, high-amplitude facies containing small channels and chaotic, high-amplitude facies produced by local mass transport complexes; at least four mass transport complexes within the basin floor fan have been identified at the Ursa Field alone.

The basin-floor fan, composed of the MC Blue Unit, is overlain by a slope fan complex comprising a succession of four canyon-channel-levee systems (named Ursa, Southwest Pass, Old Timbalier, and Young Timbalier), numerous mud-rich mass transport complexes, and hemipelagic mud. Some of these canyon systems can be traced from incised valleys of the Mississippi River on the continental shelf to fan channels on the Mississippi fan. Channel sands (massive to thick-bedded) and levee sands (thin-bedded) of the Ursa Canyon and Southwest Pass Canyon have been penetrated in several locations. The slope fan-basin floor fan boundary is obscured over much of Greater Mars-Ursa area, partly by channel incision, but more importantly by paired, channel-margin slides, which are most extensively developed subjacent to Southwest Pass Canyon. The Southwest Pass Canyon channel-margin slides comprise a 2-5 mile wide belt of rotated slide blocks dipping away from the channel axis. The P1 SB typically serves as the detachment surface for the Southwest Pass Canyon slide complex; this detachment level is several hundred feet deeper than the channel thalweg. The geometry of these slide complexes clearly indicates that they were triggered by channel incision. Similar slide complexes occur on the Mississippi fan, where they have previously been misinterpreted as mass transport complexes predating their corresponding fan channels.