Fluid Migration and State of Stress Above the Blue Unit, Ursa Basin: Relationship to the Geometry of Injectites
Authors: Moore J. Casey , Iturrino Gerardo J. , Flemings Peter B. , Hull Ian , Gay Aurélien 
Published: October 2008

Sands penetrated at shallow depths (&lt; 1 km) in the Gulf of Mexico can flow to the surface and undermine seafloor installations. Here we explore aspects of natural fluid expulsion structures from these "shallow-water sands". Highresolution seismic data shows no obvious sand injection structures from the Blue Unit, a shallow-water sand, in the Ursa area, but perhaps nascent dike formation. Also, channel sands stratigraphically above the Blue Unit are overpressured, with a possible pressure source in the latter. In the Ursa area the maximum principal stress is vertical and constrained by the integrated unit weight of the sediments at IODP Site U1324. The stress necessary to form hydrofractures (the minimum principal stress plus negligible tensional strength of the sediment) is about 85% of the maximum principal stress indicating an approximately isotropic state of stress. High Poisson's ratios (0.46-0.49), determined from wireline compressional and shear wave velocities at Site U1324, are also consistent with a nearly isotropic state of stress at shallow depths. Estimated maximum fluid pressures are about 80% of the total overburden stress, less than necessary to hydrofracture the sediment. Therefore, the fluid pressure is insufficient to form sand injectites. The formation of sand injectites, as observed in the geologic record, may require very high fluid pressures, approaching lithostatic values. Diverse orientations of ancient injectites support the necessity of very high fluid pressures and a nearly isotropic state of stress.

Introduction

Drilling operations in the Gulf of Mexico, especially in deep water, have encountered unconsolidated and overpressured sand at shallow depths. Once penetrated these sands flow into the wells, washout, erode around the exterior of casing strings, and locally flow to the surface [1]. Surface features associated with this ?shallow water flow' behavior include cracks, faults, and gully-like features [2]. In areas where this behavior has occurred, the seafloor appears to be undergoing subsidence associated with sediment loss at depth. In the Ursa drilling area (Figure 1) the shallow-water sands are overpressured which initiates the flow into the boreholes [1, 3].

The shallow-water sands in the Ursa area readily flow into boreholes and to the surface if the borehole is normally pressured (i.e. the hole is uncased or an underbalanced mud is used). Therefore, an obvious question is whether there are any natural injections from the shallow-water sands or other evidence of fluid flow. Given the overpressured nature of the shallow-water sands [3], naturally occurring injections and other fluid migration features could be expected. These "injectites" are common in some petroleum systems where they complicate sand reservoir geometry and have important implications for hydrocarbon production [4]. Because fluid pressure and sand geometries are well-constrained, the Ursa area may be an important natural laboratory to ascertain the limiting fluid pressure and stress conditions for the occurrence of injectites and other fluid flow features.

Fluid Migration and Sand Injection from Blue Unit Sand in IODP Drilling Area

During the last 70 ka, rapid deposition, directly south of the Mississippi River accumulated a sand and mud sequence, the Blue Unit [5].