Neil Sherwood

Chemical examination of some petroleum source rocks by laser pyrolysis-mass spectrometry and flash pyrolysis-gas chromatography/mass spectrometry

Abstract A combination of laser desorption and electron impact (LD-EI) ionisation enables a novel mass spectrometric study of a suite of potential petroleum source rocks and related samples. Comparision of the LD-EI mass spectrometric data with gas chromatography mass spectrometric data from flash pyrolysis reveals that structurally relevant products are detected from both types of experiments. The relative concentrations of the predominant products allows assessments of the aliphatic and aromatic contents of the organic matter in the samples. The detailed information provided directly by LD-EI mass spectrometry, without gas chromatography, demonstrates that the technique offers a rapid, comprehensive method for organic geochemical analyses of preserved organic matter in rocks.

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.

Organic matter network in post mature Marcellus Shale: Effects on petrophysical properties

Shale samples of the Marcellus Shale from a well drilled in northeastern Pennsylvania were used to study diagenetic effects on the mineral and organic matter and their impact on petrophysical response. We analyzed an interval of high gamma ray and anomalously low electrical resistivity from a high thermal maturity (mean maximum vitrinite reflectance > 4%) part of the shalehgas play. A suite of microanalytical techniques was used to study features of the shale down to the nanoscale and assess the level of thermal alteration of the mineral and organic phases. The samples are organic rich, with total organic carbon contents of 3–7 wt. %; the vast majority of the organic matter was identified as highly porous pyrobitumen. Matrix porosity is also present, especially within the clay aggregates and at the interface between rigid clasts and clay minerals. Mineral- and organic-based thermal maturity indices suggest that during burial the sediment had been exposed to temperatures as high as 285°C (545°F). Under these conditions, the residual, migrated organic matter assumed a partially crystalline habit as confirmed by the identification of turbostratic structures via electron microscopy imaging. Experimental dielectric measurements on organic matter–rich samples confirm that the anomalous electrical properties observed in the wire-line logs can be ascribed to the presence of an electrically conductive interconnected network of partially graphitized organic matter. The preservation of porosity suggests that this organic network can contribute not only to the electrical properties but also to the gas flow properties within the Marcellus Shale.