Microbiostratigraphical Studies of the Gas Field in Chiba Prefecture, Japan

The gas field in Chiba Prefecture occupies the main part of the gas-producing area of the southern Kanto region, and is one of the largest gas fields in Japan. Stratigraphically the majority of the gas-producing strata in the area are restricted to the marine Kazusa group which consists of 10 formations. These formations are mainly composed of alternations of sandstone and siltstone, and were deposited in a large sedimentary basin of Pliocene to early Pleistocene age. The writer had studied the Kazusa group from the view points of microbiostratigraphy and economic geology, for several years examining many cores collected from the gas wells and surface cuttings in Chiba Prefecture. The purpose of the present article is to summarize the lithologic variation, microbiostratigraphy, paleoecology, depositional history and some economic problems of the field. From the detailed study of the foraminiferal faunas and rocks collected from many wells in this area, the following facts were recognized : 1. Eleven foraminiferal zonules are discriminated in the Kazusa group of the area. The sequence and dominant species of these zonules are shown in Table 1. 2. The lithologic character of the Kazusa group in the Kujukuri coastal plain changes from southwest to northeast. However, the biofacies of the Foraminifera are stable, and correlation of the wells by means of the foraminiferal zonules is easy. 3. The thickness of the formainiferal zonules generally converges toward the northeast in the Kujukuri section, and some changes in the dominant species of the assemblages characterizing the zonules are recognized. Especially, the Lower Kokumoto faunule is not discriminated at the northeast end of this section, because of the faunule thinning out. 4. As shown in the isopach maps (figs. 11-15) of each formations of the Kazusa group, the center of deposition of the group migrated from the southern part of the Kujukuri coastal plain area towards the northwest, during the deposition of respective formations. 5. The isolith maps (sandstone : figs. 16-19) and sandstone percentage maps (figs. 20-24) of these formations show the following facts. The sandstones are developed regularly in the Ohara, Namihana and Katsu-ura formations, but irregularly in the Kiwada, Otadai and Umegase formations. In the Kokumoto formation, the sandstone again shows a regular distribution pattern. From these facts the writer has concluded that the sandstones of the Ohara, Namihana, Katsu-ura and Kokumoto formations were deposited in shallow water under the normal sedimentary process, but the ones of the Umegase and Otadai formations were secondarily transported to deep water. 6. The paleoecologic conditions of these formainiferal zonules may be explained from the Recent distribution of Foraminifera around Japan. The biofacies maps (figs. 7-10) summarize s the regional variation of these paleoecologic conditions. The depositional history of the Kazusa group is as follows : At the beginning of deposition of the group, its sedimentary environment was of relatively shallow water and the basin unstable. During the depositional stages from the Kiwada to Umegase formations, the environment was relatively deep and the basin was in a stable bathyal condition. During the Umegase stage the sea water attained the maximum coverage in Chiba Prefecture. Regression might have begun with the Kokumoto stage. This may be explained from the dominance of neritic species of the Kokumoto and its superjacent formations. The foraminiferal assemblage of the Kasamori formation indicates the most shallow water habitat throughout the deposition of the Kazusa group 7. The argillaceous sediments in the stable bathyal sea bottom are most favourable as the mother rocks of natural gas. The siltstone layers of the Otadai and Umegase formations are good examples of them. On the other hand, the sandstone layers intercalated in them are very good gas-reservoirs, because they are composed of sand grains excellently sorted and graded as a result of secondary transportation such as by turbidity currents. The natural gas accumulations of high potentiality in these formations are on the east side area of Chiba City. In the west side area of Chiba City, the foraminiferal assemblages of these fornrations indicate shallow water-environments, and the geological characters of these formations are somewhat different from those of the east side. The different characters between the east and west side areas of Chiba City cause the different mode of accumulations of natural gas