Abstract: Natural Gases and Their Use in Exploration and Production

Martin Schoell

New isotope analysis techniques together with a better understanding of formation and migration of natural gases and better interpretive techniques opened new applications for gas isotope analyses in exploration and production. This presentation will give an overview of fundamental principles in gas geochemistry, which will be explained using worldwide case histories. In addition, the lecture will discuss applications that are of special interest to explorationists and production engineers.

Exploration Applications

Determination of Gas Type: Isotope analyses allow for an accurate genetic characterization of natural gases (bacterial, thermogenic, oil related, deep dry). The fundamental processes that control the isotopic composition of a gas are maturation and mixing. Modern interpretive techniques allow the deconvolution of mixing processes and evaluation of the genetic types encountered in a gas province or in a gas field.

Maturity Estimate: The combination of isotopic signatures of various compounds in a gas is used in empirical cross plots for assessment of the maturity of the source of a gas. Several examples show that different methods give different results. Methane isotope variations are still the most reliable for maturity assessment.

Prediction of Gas vs. Oil: Gas isotope signatures in oil-related gases often differ from those of coal-related gases. Using regional calibration databases one may be able to differentiate and predict oil- vs. coal-related gases that are not associated with oil.

CO₂ risk assessment: The amount of CO₂ in natural gases correlates often with other properties in gases such as the isotopic composition of methane. These relationships can be used to predict CO₂ in natural gases.

Production Applications

Natural gases vary in their chemical and isotope composition as a function of their formation and migration history. Compositional and isotopic variations are often caused by mixing of two or more compositionally and isotopically different gases. Isotopic properties in gases can be used to determine the mixing ratio of the two end members and/or calculate the composition of the end members from different mixing ratios. The variation of isotopic properties of gases within a continuous reservoir are generally small but can be significant between fault blocks of one reservoir or between unconnected but closely stacked reservoirs. These inter-reservoir variations can be used to help solve many of the problems occurring during gas field development and operation.

Fingerprinting of Reservoir Gases: Isotope analyses of C₁ to C₄ compounds in natural gases provide a precise correlation of gases through a comparison of their compound isotope patterns (isotopic Fingerprints).

Reservoir Identification: Gaseous compounds in reservoirs of most oil and gas fields exhibit differences between individual reservoirs due to different Idling and mixing histories. In many oil and gas fields, each reservoir can be differentiated from another, but gases within a single reservoir are very similar. Isotope analyses of gases could be helpful in such cases to identify the production zone in new completions.

Reservoir Compartmentalization and Fault Block Mapping: Numerous case histories show isotopic properties of gases change across sealing faults. Thus, gas isotopes can be used to indicate compartmentalization into fault blocks. Such variations can be used by field geologists to better define faults and reservoir configurations.

Production Allocation: Isotope analyses in commingled production could be used to allocate contributions from individual sands if isotopic differences exist between the gases from the contributing reservoirs. Theoretical mixing curves for two-end-member scenarios and three-end-member scenarios will be discussed.

Production Monitoring of Conventional and Horizontal Wells: Production monitoring is a new application of gas isotope analyses that could be particularly valuable for horizontal wells. Isotope variations in the produced gases would reflect variations in the input of different reservoirs.

Variation Trends in Oil and Gas Fields: Large gas fields are generally filled by multiple charges of compositionally different gases, resulting in lateral variations in large gas fields (Troll, Yacheng, Hugoton). This is of particular interest if the variations affect the value of the production. Isotope analyses of gases may allow scientists to predict the amount of co-produced fluids or may be related to the amount of inert compounds in the gas, both of which affect the value of production. Systematic mapping of these variations may allow establishment of trends and compartments of more or less valuable parts of the field. With this information production facilities can be designed for optimal exploitation of the field.

Mud Gas Isotope Analyses While Drilling: Isotope analyses can be performed on mud gases that are collected in the mud-logging unit during drilling of a well. Mud gas isotope analyses provide information prior to testing. An example from an offshore well will be discussed where mud gas isotope analyses successfully predicted condensates in gas reservoirs.

AAPG Search and Discovery Article #90948©1996-1997 AAPG Distinguished Lecturers