Abstract
Reservoir rocks of this province are composed of sandstone, tuffaceous sandstone, tuff, tuff breccia, agglomerate and lava. It is very striking that many of reservoirs were originated from volcanic rocks. Pyroclastic rocks of liparite or rhyolite, dacite and andesite in the "green tuff" have been proved to form excellent reservoir rocks.
Characteristics of volcanic rock reservoir are that they have effective pores originating from fissures and/or vugs besides intergranular pores. This is indicated by comparison of porosity values from Sonic Log and Formation Density Log on sandstone reservoir and volcanic rock reservoir.
The writer attempted, in this paper, to review the resuls of exploratory wells and the state of hydrocarbon production from the "green tuff" in some areas where much efforts have beer devoted to exploration of the "green tuff".
In oil and/or gas fields which are producing hydrocarbons from the "green tuff", lava and tuff breccia constitute reservoir rocks, but tuff, especially fine or bentonitic tuff which surrounds lava and tuff breccia, plays the role of cap rock.
Large hydrocarbon deposits in the "green tuff" have been found in the area colse to abundant source rocks. Petroleum or protopetroleum should have migrated into lava or tuff breccia through tuff. The time of migration should be just after tuff was deposited, when tuff had not yet been compacted or altered so severely so that petroleum could not permeate it.
Fumdamental requisites for hydrocarbon deposit in the "green tuff" are as follows.
(1) Contemporaneous source rocks should exist close to the "green tuff".
(2) "Green tuff" should have formed relative high at the time of migration and it has developed into anticline or anticlinal structure.
(3) Good porosity of any kind should exist in the "green tuff".
(4) Fine or muddy tuff in the "green tuff" should have turned into cap rock contemporaneously with the progress of hydrocarbon migration.
