VOLCANOSTRATIGRAPHY OF NANGERANG REGION AND SURROUNDING AREAS, A PREELIMINARY STUDY OF POROSITY IN TERTIARY VOLCANIC ROCKS MUHAMMAD IMRON HASANI, MIRZAM ABDURRACHMAN, IDHAM ANDRI KURNIAWAN, TAUFIK ISMAIL
INSTITUT TEKNOLOGI BANDUNG
Abstract
Recently many volcanic reservoirs are produced and become proven reserve for hydrocarbons. However, many of these exploration process in volcanic environment faces major obstacle in understanding the reservoir itself. Being different from sedimentary reservoir, volcanic reservoir has different approach of study and character. In order to get more understanding, analogs outcrops are being used. Analogs are being used to help understand the process of porosity formation and their diagenetic process. The location of outcrops analog is located in Nangerang, Sukabumi Regency
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The methods are volcanostratigraphy and petrographyical analysis. Volcanostratigraphy analysis is carried out to determine the position, geometry, and deposit evolution at its central, proximal, medial, and distal positions. 23 samples of petrography are carried out to determine the porosity type and their diagenetic process.
The result shows 11 (eleven) volcanostratigraphy units, consisting of Basement Rock (1 unit, sedimentary rock, Jampang Khuluk (1 unit, composite of lava and pyroclastic breccia), Nangerang Khuluk (2 units, tuff and composite of lava and pyroclastic breccia), Bantarpanjang Khuluk (6 units, pyroclastic breccia) and Resedimented Unit (1 units, volcanic breccia). Using references and foraminifera fossils analysis, the age of Basement Rock is Early Miocene Age and equivalent with Jampang Formation. Other volcanic units are estimated to be Late Miocene-Pliocene based on previous studies.
The petrographical analysis shows that the matrix of pyroclastic breccia (flow deposit) and tuff (fall deposit) in all samples have primary porosity, composed of intergranular or intragranular or both with the total of visible porosity less than 5%. The appearance of dissolution/vuggy enhances the porosity of samples with the total of visible porosity 5-18%. The genesis of dissolution in the samples comes from weathering and alteration. However, some of samples shows that the alteration process decreasing the visible porosity because of quartz mineral filling.
