Some Palaeozoic (Permian) and Mesozoic reservoirs of Abu Dhabi contain variable amounts of inorganic gases, namely hydrogen sulphide, carbon dioxide, and nitrogen. The vertical and horizontal distribution of these gases vary considerably owing to various factors such as the proximity to the source area, hydrocarbon and other basinal fluids migration pathways, burial depth, variation in geothermal gradient, and in the case of hydrogen sulphide, the presence of anhydrite. The inorganic gas concentrations together with the impact of their existence on reservoir quality are discussed in this paper.
Integrated petrographic and gas data analyses indicate a proportional relationship between secondary calcite cementation and hydrogen sulphide concentration. Calcite generated as a by-product of the thermo-chemical reaction between anhydrite (CaSO4) and organic gases mainly methane (CH4), played a significant role in damaging the porosity in the studied reservoirs. A direct effect of this phenomenon is witnessed in the Khuff Formation in Abu Dhabi (Worden et al., 1995). This process probably contributed to the marked deterioration of reservoir quality in some of the Thamama zones at NE Bab field. Calcite rich basinal fluids, originated in the Falaaha trough and generated from the thermo-chemical reaction of methane and anhydrite, migrated laterally through the Asab Oolite and then vertically into the Lower Cretaceous reservoirs. This process, among others, has contributed to the deterioration of the petrophysical characteristics of these reservoirs.
Carbon dioxide, which is dissolvative, in formation water, created a weak acid that provided additional microporosity. In the Habshan Formation, the CO2 content increases with depth and as a consequence, the porosity in these reservoirs was enhanced due to leaching caused by the dissolved carbon dioxide.
Approximately 31% of the World's proven gas reserves are located in the Middle East, about 13% of which exists in Abu Dhabi area mainly in Bab, Asab, Hail, Bu Tini/Ghasha, and Umm Shaif fields (Fig. 1a). Lower Cretaceous reservoirs contain 80% of the oil accumulation and 50% of the gas, while the Upper Jurassic sequence contains some 15% of the oil and 30% of the gas in the Emirates. Most of the fields that contain Thamama gas zones are located in central Abu Dhabi juxtaposed to the Falaaha trough (Hawas, 1995).
This study is concerned with the distribution of inorganic gases in Abu Dhabi fields and the impact of their presence on reservoir quality. The distribution of these gases, within the same reservoir (e.g., in Bab-Bu Labyad fields), is variable both with depth and the lateral extent of the reservoir. Hydrogen sulphide is an undesirable component in gas reservoirs. It reduces the economic value of natural gas in addition to the fact that it can be extremely hazardous and cause severe damage to production equipment (Worden et al., 1995). The present study, shows that the existence of high concentrations of thermo-chemically generated hydrogen sulphide is an indicator of reservoir quality deterioration. Most of the hydrocarbon bearing zones within ADCO's concession show a prevailing down structure deterioration in reservoir petrophysical characteristics. Several diagenetic processes may have contributed to this phenomenon, including chemical compaction that intensifies with the gradual increase in the depth of burial and the changing subsurface water saturation, giving rise to cementation and stylolitization. Oswald et al. (1995) suggested that in Kharaib formation in Sahil field, pressure solution along stylolites accompanied with calcite cement re-precipitation is the main process causing the deterioration in reservoir quality of some of the Thamama zones. However, some of these zones namely in north-east Bab. Bu Labyad, and Al Dabb'iya exhibit a different diagenetic trend. A marked northeast deterioriation in reservoir quality is evident in Bab field, while in Bu Labyad and Al-Dabbiya fields the deterioration is in the southwest direction. A northward decrease in poroperm is also observed in Asab oolites. It is believed that both normal diagenetic processes, and souring process contributed to the northeast reduction in porosity (Bab field).