Mohinudeen Faiz

Thermal maturity and suppressed vitrinite reflectance for Neogene petroleum source rocks of Japan

The northern part of Honshu Island contains the major petroleum resources of Japan. The source rocks mainly comprise a sequence of Miocene marine mudstones that are overlain in places by thick Pliocene and Pleistocene sediments. In the past, mainly vitrinite reflectance analyses have been used to evaluate thermal maturities of these rocks. However, vitrinite reflectance suppression caused by compositional variation of vitrinite is common in marine-deposited rocks, and therefore, modeled estimations of the extent of petroleum generation from the Japanese sequences could be in error. Fluorescence alteration of multiple macerals (FAMM) analysis is a method that aids in solving the problem of vitrinite reflectance suppression and gives improved evaluations of thermal maturity. Combined vitrinite reflectance and FAMM analyses of potential source rock sequences intersected by the Ministry of International Trade and Industry (MITI) Shin-Takenomachi and MITI Nishi-Kubiki wells of the Niigata Basin and the MITI Honjo-Oki and MITI Yuri-Oki-Chubu wells of the Akita Basin show that vitrinite reflectance suppression is common in the Neogene source rocks. This leads to major differences between the depth profiles for vitrinite reflectance and for FAMM-derived, equivalent vitrinite reflectance. On the basis of vitrinite reflectance, the thermal maturity and hence, the petroleum source rock potential is underestimated for the Miocene Noudani Formation of the Niigata Basin and the Pliocene Funakawa and Miocene Onnakawa and Nishikurosawa formations of the Akita Basin. The new thermal maturity data indicate that these formations would have generated more oil than previously thought, such that petroleum prospectivity for areas including these sequences should be reassessed.

Higher Hydrocarbon Gases in Southern Sydney Basin Coals

Coal seams of the Sydney Basin contain large volumes of gas, mainly methane (CH4) and carbon dioxide (CO2) with subordinate amounts of heavier hydrocarbons (C2+). The desorbable gas content of the Sydney Basin coals ranges up to about 20 m3/t, and its abundance is mainly related to depth and geological structure.

NATURAL ACCUMULATION OF CO2 IN COALS FROM THE SOUTHERN SYDNEY BASIN—IMPLICATIONS FOR GEOSEQUESTRATION

The southern Sydney Basin is an ideal natural analogue for CO 2 geosequestration because of the widespread CO 2 occurrence, extensive data sets available and general knowledge of gas distribution. The CO 2 mainly occurs adsorbed in coal, incorporated into carbonate minerals and dissolved in formation water. On this basis, an area of -900 km 2 has been chosen for detailed examination. Gas in the coal seams of this area contain mainly CH 4 and CO 2 , the CO 2 content ranging from <1 to 20 m 3 /tonne. The 13 C values indicate multiple sources including thermal maturation of coal, microbial alteration of pre-existing gases and magmatic activity as the main source. The highest concentrations of adsorbed CO 2 occur mainly in anticlines and between -300 and 600 m possibly reflecting inverse relatonship with CO 2 solubility in formation water. Carbon dioxide appears to have migrated from the deep-seated magmatic sources along faults and permeable strata, towards structural highs and stratigraphically shallower coal seams. Calculations indicate that about 78 x 10 6 tonnes of CO 2 are presently stored in coaly intervals in the study area. Assuming a storage capacity of 20 m 3 /t for these coal seams, the total CO 2 storage capacity for the coaly intervals is -880 x 10 6 tonnes. Using the study area as an analogue for enhanced coal seam methane production, 175 x 10 6 tonnes of CO 2 could be stored, assuming a 50% CH 4 recovery factor and an average CO 2 sorption capacity 1.5 times that for CH 4 .