九十九里地域におけるガス水比およびヨウ素濃度を規制する要因

特に,海底扇状地堆積物および断層が果たす役割について

抄録

We examined control factors of regional variation of gas-water ratio and iodine concentration based on the integrated evaluation of the subsurface submarine-fan distribution of Umegase and Otadai formations, the location and displacement of N-S trending faults developed in the Kujukuri District along the Pacific Coast of Chiba Prefecture, central Japan. The main results are as follows; 1) Umegase and Otadai formations which consist of turbidite successions as main reservoirs are the thickest nearly at the southwestern margin in the study area and have elongated northeastward as far as about 20 kilometers, i.e., the thickness of the formations decrease gradually northeastward, are considered as ancient middle to lower fan deposits formed under channel-lobe system based on the electric logging patterns. 2) Regionally, gas-water ratio increases as well as Umegase and Otadai formations thicken, and the distribution of gas reservoirs with high gas-water ratio, usually known as Mobara-type gas reservoir, are well coincident with that of submarine-fan deposits. Locally, extremely high gas-water ratio tends to be located in the west side of main eastward-dipping normal faults. These facts strongly support the idea that biogenic gas moved the southeastward, i. e., to the depocenter of the submarine-fan deposits and to the up-dip direction, and then it was trapped in the west side of major N-S trending normal faults. 3) The regional change of iodine concentration (I/Cl) is basically concordant with the thickness of the submarine-fan deposits. Iodine concentration tends to be highest near the depocenter of submarine-fan or turbidite deposits in open sea environments. This fact suggests that organic matters both of continental origin such as plant fragments in turbidites as absorption materials and of marine origin such as planktons as original iodine supplier are important to produce high iodine concentration in brine.