Arc-related volcanism activity in Java subduction system has been started since Eocene. This activity
produces thick sequences ofvolcaniclastic rocks. Volcanic activity during Miocene in Southern
Mountain marked by ancient volcanic that produced volcanic deposits that compose Semilir Formation.
Outcrop in Ngoro-oro, Patuk, Gunung Kidul, Special Region of Yogyakarta, has an ideal dimension
and various lithology to identify petrophysical properties such as reservoir space. This paper uses
detailed stratigraphic measurement with 1:10 scale which determines the facies distribution and
analytical petrography to determine the reservoir characteristics based on mineralogy, texture and
diagenesis process. The results shows that the study area consist of lapilli-vitric stone, greywacke and
vitric-crystal tuff. Volcanic rocks in Semilir Formation, Gunung Kidul area develop into three
reservoir space types : primary pores, secondary pores and fractures pores. This formation of reservoir
space went through diagenetic process consist of cooling and solidification stage and epidiagenesis
stage. Cooling and solidification stage include fragmentation, crystallization, differentiation and
solidification, epidiagenesis (Re-Construction) stage include weathering, leaching, fluid dissolution
and tectonism. Primary pores were formed during volcanic active period and solidification period,
marked by volcanic rocks with various composition, grain size and contraction fractures. Secondary
pores and fractures pores formed at epidiagenesis (Re-Construction) stage, with distinct features as
weathering-leaching period and burial period, marked by micro-structure, solution pores fracture along
cleavage face and devitrification pores. Volcanic rocks diagenesis compelixity are the major factors
that control of complexity and diversity in reservoir space types.
Kaa kunci : diagenesis, volcanic rocks, porosity, semilir formatio
