THE VALUE OF PETROGRAPHY IN A GEOCHEMICAL ERA

Traditional petrographic techniques combined with geochemical studies provide insights into the episodic nature of fluid migration in two fine-grained Cretaceous limestone reservoirs, one from Skjold Field in the Danish North Sea and the other from the Shuaiba and Kharaib Formations in offshore Qatar.

Petrography and fluid-inclusion investigations of chalks from the Skjold Field indicate that successive zones within calcite fracture fills have water- and oil-filled fluid inclusions, sometimes with a corrosion zone between them. These variations are interpreted to reflect oil filling and overpressuring of the structure, followed by failure of the seal, with loss of overpressuring and flushing of oil charge by hot, saline brines. After pressure dissipation, calcite cementation under aqueous conditions helped to heal the fractures and allowed gradual repressurization and refilling of the structure with hydrocarbons. Other evidence of loss of overpressures and influx of hot brines include the presence of oil trapped in very low porosity and permeability chalks, alteration of stable isotopic ratios, anomalously high fluid inclusion temperatures, significant dolomite in some horizons and the presence of celestite in fracture fills.

The Shuaiba and Kharaib carbonates are characterized by extensive secondary porosity, including biomolds, leached intergranular calcite cements and solution-enlarged fractures and vugs. Early diagenesis was dominated by marine cementation, aragonite dissolution or neomorphism, calcite cementation, multi-stage dolomitization and small-scale fracturing. The main phase of porosity development postdates marine and eartly burial diagenesis and appears to be related to large-scale throughput of highly corrosive pore fluids. Corrosion of earlier cements, development of solution-enlarged fractures and vugs and the presence of two-phase fluid inclusions in secondary cements support a relatively late origin of this porosity. Abundant hydrocarbon-filled inclusions in some cements indicate involvement of hot brines that may have just preceded or accompanied hydrocarbon migration. A likely origin for such fluids is from nearby Zagros or Oman overthrust belts.

In both examples, petrographic observations improved interpretations of complex or ambiguous geochemical data.