Yoshihiro Ujiié

Brightness of pollen as an indicator of thermal alteration by means of a computer-driven image processor : statistical thermal alteration index (stTAI)

Abstract The brightness, or gray level, of pollen of Pinus, Podocarpus, Abies, Picea and Tsuga from the Neogene sediments in northern Japan was measured using a transmitted-light microscope with a computer-driven digital image processor. The mean value of the modes for the complete array of the indigenous pollen in a rock sample was called here “the statistical thermal alteration index” (stTAI). In the present investigation an inverse relationship between stTAI and vitrinite reflectance ( R O ) in sediments was found. The application of stTAI to samples from three boreholes indicated a decreasing trend with depth. By using this trend, threshold values of intense oil generation can be evaluated (145–110). Therefore, stTAI can be a useful parameter for determining organic maturation and for identifying the threshold zone of intense oil generation.

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.

Petroleum Source Rocks from the Neogene Tsugaru Basin, Northern Honshu, Japan

The purpose of this study is to evaluate possible petroleum source rocks in the Neogene Tsugaru basin. Evaluation is based on three criteria: organic richness, organic maturity, and organic type. One hundred forty-one samples of mudstones from seven transects were analyzed. Samples from the middle Miocene Kodomari Formation and younger sections generally contain more than 0.5% total organic carbon and amorphous organic matter, but are thermally immature. Samples of the lower Miocene Odose and Isomatsu formations obtain maturities equivalent to the threshold of intense oil generation, but do not contain sufficient type or quality of organic matter to be considered an oil source. Although this study failed to identify an effective source rock, it does suggest that a possible effective source may exist off the west coast of Honshu and in the eastern part of the Tsugaru Peninsula. In these areas, organic maturation of the possibly organic-rich Neogene sediments may have proceeded further than in regions investigated in this study as a consequence of a Neogene sedimentary package that is more than 2000 m thick.