It is one of the most important factor for exploration of energy resources to evaluate the thermal history of a sedimentary basin. A common method for the evaluation of thermal history is measurement of maturity of organic matter. The recent advances in the field of mineral geothermometry are remarkable. The outline of mineral geothermometer in low temperature is given in this paper. Two topics are mentioned in detail. The one is the application of fission-track dating to the thermal history of rocks in sedimentary basin. The other is the progressive transformation of smectite to illite via mixed-layer illite/semectite. The illitization is strongly controlled by temperature, and also influenced by several factors, such as kinetics, potassium availability, pressure, and fluid and rock composition. Some examples in the Niigata Neogene basin are briefly described.
A possible mechanism of a hydrocarbon trap in the Neogene porcelanites in northern Hokkaido, Japan is discussed through petrographic and pore throat size analyses. In the studied two wells, quartzose porcelanites with lesser amount of clay and located at shallower depth have a higher reservoir quality associated with hydrocarbon shows and gas production. It indicates that matrix pores as well as fractures play an important role in the hydrocarbon trap in siliceous rocks. In the Yurihara Oil and Gas Field in northeastern Honshu, another field with quartzose porcelanites reservoirs in Japan, opaline porcelanites just above the silica mineral opal-CT/quartz transformation boundary act as cap rocks. In this studied area, however, the boundary is too shallow for opaline porcelanites to behave as cap rocks because the confining pressure is insufficient, and hydrocarbon traps are formed in the quartz zone substitutively; porcelanites with much clay minerals and smaller pore throat are cap rocks and those with lesser amount of clay minerals and larger pore throat form hydrocarbon reservoirs.
The present paper proposes a fast algorithm for 3-dimensional depth transform of seismic prospecting data before stack by use of equi-travel time planes. A reflection point on reflector for each pair of source and receiving point is located on an ellipsoid of which foci are source and receiving point. Consequently, reflector is determined by drawing a common tangent plane to all ellipsoids. As a result of 3-dimensional model studies, it was concluded that this algorithm reconstructed the structure with high accuracy, the short calculation time and the requirement for small size of core memory in computer.