Cuttings materials from Katagai Gas Field in the vicinity of Nagaoka City, Niigata Prefecture, were analyzed for biological markers (triterpenoid and steroid hydrocarbons) in order to evaluate their validity as maturity indicies, with the following results. (1) Isomerization of both steranes and triterpanes are recognized from immature to oil generation zone. (2) The most reliable criteria in the immature zone and the oil generation zone are inferred to be the isomerization of hopane nucleus and the epimerization of stigmastane side chain, respectively. (3) Change of monoaromatic steroid hydrocarbons into triaromatics seems to occur from the immature to oil generation zone. (4) Cracking reactions in the side chains of C26, 27, 28-triaromatic steroid hydrocarbons into C20, 21-triaromatics appear to take place from the late stage of oil generation zone to the overmature zone. This reaction may be useful as a maturity index of crude oils or gas condensates, which cannot be measured for vitrinite reflectance.
Eleven calcareous nannofossil biohorizons are recognized in the Quaternary sequences at six sites drilled in the Northeast Atlantic Ocean during DSDP-IPOD Leg 94. Correlation of those biohorizons with the magnetostratigraphy is established, and ages of all datums are estimated by interpolation between magnetic reversals (TAKAYAMA and SATO, 1986). The calcareous nannofossil and the foraminiferal assemblages of the Nishiyama and the Haizume formations distributed in the Oginojo area, near the type locality of the Haizume Formation, Niigata district are described, and five calcareous nannofossil biohorizons are recognized. Based on the above mentioned results of DSDP-IPOD Leg 94, the middle Nishiyama through the Haizume formations are Pleistocene in age and can be correlated with the interval between the Olduvai and the Jaramillo events of the magnetostratigraphy. This age assignment agrees neither with the magnetostratigraphic interpretations (YOSHIKOSHI, 1983) nor with the fission track datings (MURAMATSU, 1983), which were undertaken in the Oguni area 25 kilometers south of the present area.
A static formation temperature (TBU) is estimated from bottom-hole temperature (BHT) circulating time (tk) and the elapsed time after circulation stopped (Δt), using a Horner-plot method. TBU is compared with reservoir temperature measured in conjunction with bottom-hole pressure survey (TBHP) or with drill stem test (TDST) in the oil and gas fields with various geothermal gradients. The Horner-plot method gives a reliable static formation temperature in regions of low geothermal gradient. The TBU in high temperature wells is lower than TDST and TBHP. The difference between TBU and TDST or TBHP in the regions of the high geothermal gradient is caused basically by the equation Eq. 1 of the Horner-plot method. BHT=Tf-Clog(tk+Δt)/Δt Eq. 1 Where, Tf is true formation temperature. According to DOWDLE and COBB (1975), comparative analytical studies of the temperature buildup and pressure buildup using the diffusivity equation showed that the two method are not completely analogous. And, Eq 1 is not correct theoretically. MIDDLETON (1982) has proposed that the temperature behavior at the bottom-hole was presented by the equation Eq. 2. BHT=Tm+(Tf-Tm)〔exp(-a2/4κ•Δt)〕Eq. 2 Where, borehole of radius a, thermal diffusivity κ. As compared with Eq. 2, Eq. 1 does not include main factors of the thermal diffusivity and mud temperature (Tm) in BHT stabilization model. However, the Horner-plot method is useful to estimate the reliable static formation temperature from BHT in the regions of low to middle geothermal gradient less than about 4°C/100m. A careful observation is necessary in approximating TBU at the high-temperature field.
The “Miocene Regional Basalts”, proposed here by the authors, include basalts and dolerites of the Nishikurosawa and Onnagawa stages, which are widely distributed from Aomori through Yamagata to Niigata oil and gas fields along the Japan Sea coast. Some of these basalts play an important role as oil and gas reservoir. In the present paper, we describe occurrence, mineral chemistry and bulk chemistry of basaltic rocks in the Niigata oil and gas field, especially in the Yahiko-Kakuda area and the Higashiyama area. Systematic lateral variations in the chemical composition of the Miocene Regional Basalts are not observed on the basis of the petrochemical characters of these basalts, including basaltic rocks of the Niigata oil and gas field and those from other localities in Yamagata, Akita and Aomori Prefectures. The Miocene Regional Basalts are composed of not only the island arc-type tholeiite but tholeiitic and alkali basalts which are rich in TiO2 and/or P2O5 and similar in composition to basalts of the oceanic region. It may be possible that such tholeiitic and alkali basalts of oceanic-type were derived from an upper mantle material different from the present upper mantle material in Northeast Japan arc. The origin of these basaltic magma might be derived from a mantle diapir in the same constitution as that of Iceland volcanics. The middle Miocene volcanics of the island arc-type tholeiite and of calc-alkaline series in Northeast Japan arc may have its origin in relation to partial melting of upper mantle and lower crust owing to the supply of heat from the ascending mantle diapir.
Reservoir rocks in the Green Tuff, especially in the Mitsuke oil field, Yoshii-Higashi Kashiwazaki and Minami Nagaoka-Katakai gas fields, are examined using evaluation techniques such as petrological method, core analysis, log analysis and pressure analysis. These reservoir rocks are mainly composed of subaqueous lavas of acidic volcanic rocks (rhyolite-dacite), and are classified into two types based on their characteristics of pore spaces. The first type, Mitsuke-Yoshii type, is characterized by common occurrence of mega-sized, primary vugs and fractures which play an important role in hydrocarbon production. On the contrary, the second type, Minami Nagaoka-Katakai type, mainly consists of micro-sized, secondary vugs, and its production behavior is similar to intergranular pores. The areal extent of volcanic bodies having high productivity varies from several hundred meters to about 3 kilometers in diameter. Acidic volcanic rocks occur in restricted areas, and their distribution trend is consistent with the stress field in the Nanatani stage.