ULTRA-DEEP RESERVOIR DIAGENETIC OVERVIEW - IMPLICATIONS FOR FUTURE EXPLORATION

Worldwide opportunities exist in the ultra-deep of sedimentary basins.Recently ultra-deep drilling has added significant gas reserves offshore of the Gulf of Mexico. The wells were drilled to 29,000 ft (8840 m), in the Norphlet Formation, Kimmeridgian age. Epigenetic calcite was later dissolved with the simultaneous dissolution and migration of hydrocarbons with chlorite coating inhibiting silica cementation Changes in pH aqueous concentrations created the diagenetic conditions derived from salt-related brines and thermochemical sulfate reduction with a vitrinite reflectance of 1 to 1.3 Ro.

Ultra-deep drilling sub-salt onshore U.S. Gulf of Mexico (25,000 ft - 35,000 ft) has significant methane gas from Cretaceous (Tuscaloosa) and Paleogene age rocks . E-logs indicate the Tuscaloosa sandstones lack feldspars and are clean, containing primary porosity. Diagenetic factors, including dissolution of feldspars, is essential in the Wilcox .The occurrence of virtualy pure methane at extreme depths is attributed the ultra clean sandstones creating inert non reactive conditions

Historically, (until 2002) ultra-deep drilling in the United States of 1676 wells exceeding 20,000 ft (6100 m), 974 (58%) are producing, of which 847 are gas wells. Oklahoma's deep wells average depth 17,500 ft (5330 m), produce 11 times more gas than wells drilled to less than 15,000 ft (4570 m) . Ultra-deep drilling in the Anadarko Basin (30,000 ft, 9100 m),and the Permian Basin (23,000 ft) contain dolostones with porosities of up to 10 percent and are due to mesodiagenetic dolomitization, with onset at 12,000 ft (3660 m) corresponding to a vitrinite reflectance of .55.Xenotopic textures result in the carbonates at ultra-deep depths.

China, the ultra-deep drilling (24,000 ft, 7300 m) resulted in giant oil discoveries in the Tarim Basin. The primary ultra-deep reservoir is a Paleozoic age intercrystalline dolomite with vuggy porosity (.9-9%). Source rocks are lagoonal carbonates and mudstones. in other basins ultra-deeply buried Triassic dolomites (21,000 ft, 6890 m) contain excellent porosities and are productive, conforming to thermal maturation models