Uncertainties in Mapping Salt Flanks with 3D Salt Proximity Survey

Abstract

Abstract This paper presents the results from a study on uncertainty analysis for mapping salt flanks with a salt proximity (SP) data set acquired in the Gulf of Mexico, where petroleum reserves are often trapped underneath overhangs of diapiric salt domes. Determination of the precise location for the salt face will help petroleum engineers to find an optimal drilling path to hit the target reservoir and allow geoscientists to obtain the best estimate for the size of the reservoir. Measurement errors of arrival times, azimuths, and vertical angles of the first breaks of SP data were estimated and their effects on the solutions of the salt face were calculated. Sensitivity tests were carried out to evaluate mapping uncertainties caused by the parameter variations of the 3D salt and sediment models. The salt face solution and associated error bars were integrated with well control data, offset VSP migration, and a surface seismic profile to interpret the steep salt flank and surrounding sedimentary rocks. Introduction It is critical important to accurately map salt flanks in the region such as the Gulf of Mexico (Fig. 1), where important oil and gas accumulations are often found around salt domes. A reliable and accurate definition of the salt face will provide both an optimal design for a well sidetrack and the best estimate for the size of a potential productive reservoir. 3D salt proximity is a seismic refraction method which has been commonly and extensively used to define the shapes of salt domes in the Gulf of Mexico 1,2,3,4,5,6,7,8. Although the method was widely used to image the salt faces, to best of our knowledge, no through studies on mapping uncertainties have been done. In this paper, we will address how accurately the salt face can be determined using a salt proximity dataset acquired in the Gulf of Mexico, USA. A deviated well was originally drilled to 10,268 ft subsea in Eugene Island Block 126 of the Gulf of Mexico, USA and it penetrated a salt body between the depths of 9324 to 9453 ft. The second well was sidetracked from the original well and drilled to a depth of 11,794 ft subsea without penetrating the salt dome. The average water depth in this area is 36 ft. A comprehensive borehole seismic survey4 was designed and acquired in the second well in order to obtain a better image of the salt face and the updip sands flanking the salt edifice. The 3C VSP data consisted of a rig-source velocity survey (VS), an offset VSP (OVSP), and a refraction salt proximity survey (SP). A map view and a cross-section of the VSP survey configuration are shown in Figs. 1 & 2. The red line represents the well trajectory. The triangles are geophone receivers in the well and the solid circles are the seismic source positions. The seismic source was a 500 in3 four-gun tuned airgun array. The rig-source velocity survey was acquired at 25 depth levels to accurately measure the vertical velocities in the sediment layers (Fig. 3). The depth migration of the offset VSP reflections4 provided an image of the updip sands and steep salt-sediment boundary, showing a structural dip of 30° for the sediment layers in the region of interesting (Fig. 2). The structural dips observed from the OVSP depth migration and the surface seismic profile (Fig. 2) along with the interval velocities derived from the VS survey (Fig. 3) can be used to build a 3D sediment model. A 3D salt model with overburden layers and caprock can also be constructed if the thicknesses and velocities of the formations are known or can be provided accurately by the client. Measurements of arrival times, azimuths (AZ), and vertical angles (VA) of the first breaks of salt proximity data can be used to calculate the coordinates (x, y, z) of salt exit points (SEP) at which the transmitted seismic energy exits from the salt edifice and enters into the surrounding sediment rocks. Thus SP survey can define a set of points in 3D space that represents the boundaries of the salt face. The 2D OVSP depth migration imaging of the steep salt boundary (Fig. 2) can provide a validation of the salt proximity result and yield an increased sense of confidence in the location of the steep salt face.