Abstract

Mississippi Canyon 348 (Camden Hills) is a recently discovered biogenic gas field in the deep-water (7200 ft) Gulf of Mexico. The gas in this field is stratigraphically and structurally trapped in a multi-story, distal turbidite channel sand complex. Although considerable work is published on biogenic gas that has been generated and trapped in shelf and deltaic depositional systems, little is understood about biogenic gas accumulations in distal, deep-water, low net sand depositional settings such as this. Given the water depth, a number of economic concerns regarding the reservoir arose during evaluation of the development and delineation wells. First, there was concern about the possible presence of wax and condensate in the reservoir gas. Detailed monitoring of the synthetic drilling mud used in the wells in conjunction with rigorous gas chromatographic analysis of the drilling mud and reservoir hydrocarbons revealed that both condensate and wax are associated with the biogenic gas.

Another engineering and economic concern in this low net sand system was the possibility of reservoir compartment-alization. Geochemical analyses of the methane revealed it to be very uniform and homogenous with respect to both its' composition and carbon isotopic signature over a vertical interval of at least 4000 ft. However, differences in isotopic signature of immature, indigenous thermogenic gases in discovery well MDT tests demonstrated vertical compart-mentalization of the various sands. Later, in the delineation well, similar gas analyses showed the main reservoir sand to be in fluid communication laterally. Early recognition of compartmentalization and the presence of wax and condensate in the reservoirs enabled Marathon to factor these considerations into the field development economics.

Introduction

Camden Hills is a biogenic gas accumulation in Mississippi Canyon 348 (Fig. 1). This field, which is in 7200 ft of water, was discovered in August of 1999. The feature drilled in the discovery well was a seismic amplitude anomaly. The trap at Camden Hills is a combination stratigraphic/structural trap although the stratigraphic portion of the trap is by far dominant over the structural component (Fig. 2). Any structural relief is due to differential compaction of the distal upper middle to middle Miocene turbidite deposits (shelf equivalent biostratigraphic zone is the Textularia W) that form both the reservoir and most of the seals at Camden Hills. Specifically, the reservoir at Camden Hills is a series of multi-story, distal turbidite channel sands. Based on core measurements taken in the major reservoir interval (Text. W-D), porosity in these sands ranges from 25 to 30% and averages 29%. Similarly, core permeability measurements indicate permeability varying between 200 to 4700 md with an average around 1500 to 2000 md. The reservoir temperature at Camden Hills is 155 degrees Fahrenheit, and reservoir pressure is 7400 pounds per square inch.

Deepwater Biogenic Gas

Examples of biogenic gas accumulations in shallow water, deltaic deposits are fairly well known. However, biogenic gas fields in deep-water, distal, low net sand systems such as the one at Camden Hills are not well documented. Prior to considering the Camden Hills biogenic gas accumulation in detail, it is first helpful to review some general concepts governing biogenic gas generation, migration and trapping. Some of the conditions favorable to biogenic gas generation are:

  1. anoxic environment,

  2. low sulfate environment,

  3. low temperature (less than about 97 degrees Celsius),

  4. adequate total organic carbon content,

  5. sufficient pore space and

  6. favorable rates of sediment deposition1,2.

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