The Use of Very High Resolution 3D Seismic Data in Conjunction with 3D Data in Evaluatina the Potential for Water Flow and Other Shallow Hazards

Abstract

The use of conventional 3D seismic data for purposes of shallow engineering evaluation has become more common place in the past three years. Conventional 3D seismic data collected for exploration purposes (4-ms sampling) has been used to evaluate the upper 5000 ft of stratigraphy in the deepwater environment (generally in water depths greater than 1000 ft). The purpose of using these data is to determine whether or not geologic conditions occur, which may prohibit or hinder exploration drilling. In many cases, these data have provided sufficient information and detail for assessing shallow drilling hazards or constraints. In some cases, very high resolution 3D seismic data may be needed to better detail potentially constraining geologic conditions. We define "very high resolution 3D seismic data" as a seismic cube with a bin size of about 50 ft by about 20 ft with a minimum of 24 fold information collected at a 0.5 ms sample rate and processed at a 1.0 ms sample. These data can be very important in defining the geometry of the potential channel or turbidity flow units where sand-prone deposits may be prime sediment bodies to produce shallow waterhand flow. Vastar Resources, Inc., collected such a very high resolution 3D seismic data set over a Federal lease block in the Mississippi Canyon area of the Gulf of Mexico known as the "Mirage" Prospect in late 1996. The data were collected over a portion of a block previously drilled by others, from which shallow water flow had occurred. The purpose of collecting this data set was to identify which zone flowed, and then to extrapolate this same zone laterally from the location using the conventional 3D data set. This paper will explain how the conventional 3D, the very high resolution 3D, well log information, and state-of-the-art software programs are used to assist in the evaluation of shallow water flow. Introduction Historically, 3D geophysical seismic data has been exclusively an exploration tool. In the last three to five years, 3D seismic data has been more commonly used to help assess shallow geologic and stratigraphic conditions for geohazards analysis, 3D seismic has in effect bridged the gap between the exploration group to the drilling and engineering groups in the petroleum industry. The information gleaned from these seismic data has been helpful to those responsible for planning drilling and production programs in the deepwater fields. Several papers presented in the last few years provide discussions on the use of these data relative to defining shallow geologic constraints (Refs. 1, 2, and 3). Others have described the use of 3D data, in conjunction with 2D seismic and MWD logging data, for determining where potential flowing water sands may exist (Refs. 4 and 5). And others have addressed the issues of how to deal with water flowing sands during drilling operations (Refs. 6 and 7). More recently operators have recognized the need to go a step further and collect really high resolution 3D seismic data to define the internal geometry of the water flow zones. The use of low volume, high frequency sleeve gun arrays in conjunction with sensitive multi-streamer arrays to collect high sampled rate data has been completed on several prospect locations in the Gulf of Mexico in the past year. Flowing water sands, from relatively shallow stratigraphic sequences, has caused more than, one headache for operators III the deep Gulf of Mexico. It has taken several years for the drilling industry to begin to understand what was occurring when circulation was being lost, when water and sands were being found to back flow up and around casing strings, and when casing began to sink or bend down hole. What are flowing water sands?