Leroy Ellis¹, Steve Uchytil², Janell Edman³

(1) Terra Nova Technologies, Dallas, TX
(2) British Petroleum, Houston, TX
(3) Consultant, Denver, CO

ABSTRACT: Hydrocarbon seal identification using mud gas isotope logging

Reservoir seals are not as well understood as either source or reservoir rocks, and evaluating and predicting reservoir seals remains problematic. Within this context, mud gas isotope logging is a promising technique for both complementing existing seal analysis methodology and empirically verifying the presence of any seal, regardless of origin.

Mud gas isotope logging is a noninvasive technique used to evaluate exploration and delineation wells. Isotopic measurements made on mud gas samples from either side of a potentially sealing interval can be used to determine the effectiveness of a seal as well as establish likely migration pathways and reservoir compartmentalization. For example, in a thermogenic gas reservoir associated with a leaky seal, gas in the overlying seal may develop an isotopic signature similar to that of the underlying reservoir gas. This leaky seal isotopic signature will be isotopically heavier and contrast with methane and ethane isotopic ratios in background shales. In contrast, an effective seal in this same thermogenic setting will have an isotopically lighter and more constant methane and ethane signal. By measuring changes in background isotopic signal of intact seals vs. seals that leak, calibration between physical property measurements of the seals and their ability to seal can be determined. To illustrate this technique, case histories using mud gas isotope data are provided for: 1) wells with leaky seals and low gas saturation in the underlying reservoir, and 2) wells with intact seals and high reservoir gas saturations.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado