Abstract

Compaction depth trends are important in drilling, basin modeling, and seismic exploration for several purposes: (1) to de-tect overpressure and hydrocarbon zones and distinguish them from seismic velocity anomalies; (2) to calculate interval velocities and depth conversion involving seismic data and Earth models; (3) to predict seismic signatures of sand-shale interfaces as a function of depth; and (4) to recognize over-compacted zones due to uplift. Several authors have studied the effects of compaction on the porosity of sands and shales (e.g., Magara, 1980; Ramm and Bjorlykke, 1994). The effects of compaction on velocity-depth trends have been provided by different authors (e.g., Al-Chalabi, 1997; Faust, 1951; Japsen, 2000). However, porosity and velocity depth trends in the shallow section are not well established. The main challenge in computing such trends is the paucity of well-log data in the shallow subsurface. Figure 1, a typical well log from the Gulf of Mexico, lacks measurements in the shallow section (< 3000 ft or ∼1000 m) due to riser-less drilling, and the log response from the deeper section cannot be used to compute the normal compaction trend due to overpressure. One way to overcome this challenge is to integrate data from multiple sources. In this paper, we compute porosity and velocity depth trends by integrating data from multiple sources including well logs, geotechnical borehole data, and core measurements from shallow sections of the Gulf of Mexico, and laboratory measurements at low effective pressure.

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