FORMATION OF GAS

BYAGBAJE MAYOWA

14/68EL001

OUTLINE INTRODUCTION

CONSTITUENTS AND CHARACTERIZATION OF

PETROLEUM GAS

GAS PRODUCT FORMED DURING HYDROCARBON

GENERATION

GAS ORIGINATING FROM INORGANIC SOURCES

OCCURRENCE AND DISTRIBUTION OF GAS IN

SEDIMENTARY BASIN

CONCLUSION

INTRODUCTION Natural gas is a hydrocarbon gas mixture consisting of principally methane, CH4, with some ethane (C2H6) and propane (C3H8), and impurities such as CO2, H2S, and N2. Natural gas is colorless.

There are different types of Natural gas namely:Dry natural gas refers to a purified product that is almost entirely methane.Wet natural gas contains compounds other than methane and ethane.Sour natural gas contains larger amounts of hydrogen sulphide, which is highly undesirable due to corrosion, and results in SO2 formation upon combustion.

CONSTITUENTS AND CHARACTERIZATION OF PETROLEUM GAS

Methane (CH4) is a major constituent of natural gas. Other components may be present: Hydrocarbons heavier than methane( mostly

ethane (C2H6), propane (C3H8), butane (C4H10) Carbon dioxide CO2; Hydrogen Sulphide H2S, Nitrogen, Hydrogen, Argon, Helium, Condensate (liquid hydrocarbons dissolved in the gas)

The main parameters used to characterize gas are the following: the methane content and that of higher

hydrocarbons expressed by the Cl/∑Cn ratio

(Methane/total hydrocarbons), the content of CO2 , H2S, N2 and other

nonhydrocarbon gases, the carbon isotope composition of hydrocarbon gas,

expressed as δ13C(%o) where δ13C1 refers to the

isotope composition of methane.

the hydrogen isotope composition of hydrocarbon gas usually expressed as δD

Fig. 1: The principal characteristics of hydrocarbon gases generated during the successive stages of Kerogen evolution. (After Stahl, 1975; Galimov, 1980; Rice and Claypool, 1981)

A systematic study of the δ13C1 measured on

methane and the Cl/∑Cn ratio allows a first and

relatively Simple characterization of gases as shown in figure 2.This illustration shows three successive stages of gas generation during the evolution of sediments: biogenic gas, generated during diagenesis, thermal gas generated during catagenesis thermal cracking gas generated during the late part of catagenesis and metagenesis.

Fig. 2. Relative abundance and isotopic composition of methane in gases from Biogenic origin (diagenesis) and thermogenic origin (catagenesis and metagenesis).

GAS PRODUCT FORMED DURING HYDROCARBON GENERATION

 STAGES OF MATURATION

HYDROCARBON PRODUCT

NONHYDROCARBON PRODUCTS

DIAGENESIS METHANE (BIOGENIC GAS)

CARBON DIOXIDE, NITROGEN, HYDROGEN SULFIDE

CATAGENESIS METHANE, ETHANE, PROPANE, BUTANE (WET GAS)

NITROGEN, HYDROGEN, HYDROGEN SULFIDE

METAGENESIS METHANE (THERMOGENIC GAS)

NITROGEN, HYDROGEN, HYDROGEN SULFIDE

Fig. 3 The Oil Window, and General Scheme of Hydrocarbons Produced (Modified after Taoist et al., 1974)

GAS ORIGINATING FROM INORGANIC SOURCESSome constituents of natural gas may be produced by purely inorganic processes. Gas originating from magmatic rocks, such as volcanic and geothermal gases may be a source of CO2, H2 and smaller amounts of CH4, H2S, N2, He and Ar. In some gas accumulations, CO2 is a major component and is probably of magmatic origin. Radioactivity is likely to be the source of helium and most argon. Helium is produced by disintegration of the radioisotopes of the uranium and thorium families. Argon results form disintegration of potassium.

Fig. 4. Distribution of the Helium content in gases from Paleozoic, Mesozoic and Tertiary reservoirs ( Tissot, 1969)

OCCURRENCE AND DISTRIBUTION OF GAS IN SEDIMENTARY BASIN

Two considerations make the distribution of gas accumulations more difficult to explain than the distribution of oil fields. Gas constituents may be derived from different sources (organic, inorganic) through different processes of generation (biogenic, thermogenic or even radioactive). They migrate more easily than oils through sedimentary column.

The distribution of quite a number of hydrocarbon gases can be reasonably interpreted in terms of stages of maturation: Diagenesis, Catagenesis, Metagenesis. The distribution of nonhydrocarbon gases is more complex, as the origin may be different from one basin to another and it can also be affected by migration. For example, CO2 is found at relatively lower depth( late diagenesis to Early Catagenesis) in South Germany but also in deep accumulation derived from late Catagenesis or even metagenesis (North Germany).

CONCLUSIONCharacterization of natural gas has to rely on prominent features such as the ratio of methane to total hydrocarbon and the isotopic composition: Carbon and Hydrogen isotopes.Biogenic methane Is generated at low temperature levels and recognized by very low isotopic ratios.Thermogenic Methane is generated at high temperature and methane can be destroyed chemically by conversion to H2S in the presence of sulfur. Nonhydrocarbon constituents (e.g. CO2, H2S) of natural gases may have an inorganic origin.