The Yuchon Group in the Late Cretaceous of Haenam and Mokpo area on the southwest coast of Korea peninsula can be divided into two Formations: (1) the intermediate volcanic Formation (Hwawon Formation), about 500m thick, (2 ) the acidic volcanic Formation (Hwangsan Formation), about 400m thick in ascending order The former comprises intermediate volcaniclastics interlayered with volcanic rocks, and red mudrock and tuffaceous sandstone indicating fluvial deposits. The latter is subdivided into the upper part (Hwangsan Tuff Member) consisting of subaerial pyroclastics and intercalated rhyolites, and the lower part (Byeongonri Member) including subaqueous volcaniclastics, lake deltaic sandstone and gravelstone, lacustrine black shale and limestone and chert, and lake turbidite sandstone. The Late Cretaceous basin including Haenam subbasin in southwest Korea was largely formed of extensional nonmarine depressions (volcano - tectonic) bounded by NE - SW sinistral fault system. The thermal maturation based on geochemical and mineralogical studies for the black shales and tuffaceous sandstones reached the late stage of oil generation zone or gas generation stage. It seems that black shales and limestones are fairly good as source rock. The porosity of potential reservoir sandstone and tuff ranges from 5 % to 11 %, but their permeability except the fractured rocks is very low (< 1md) because of fine pore throats reduced by diagenetic cementation of tuffaceous sandstones. Numerous potential traps might have been formed by the later folding and faulting along with lateral factes change and abundant mudrocks and volcaniclastics should make excellent seals.
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