Reservoir Geology and Stratigraphy

Abstract

Deep-water deposits containing variety characteristics of reservoir potential followed by the texture, facies, and geometry. The variety of composition influenced by the materials from the volcanic arc in the center of Java Island that causing the forming of volcanogenic turbidite deposits. Miocene-Pliocene deep water sediment can be found in several formations: Halang Formation and Kerek Formation in Central Java, and also Kebo-Butak Formation and Semilir Formation in East Java as the research area.

The research is focused on comparing the characteristics of each formation, particularly the information about reservoir potential by passing several methods on building vertical stratigraphic section, biostratigraphic and petrographic analysis, followed by facies association determination to interpret the depositional environment, and also quantification of outcrop data that provides additional information on reservoir quality. Halang and Semilir formation have the complete Ta-Te Bouma sequence with F7, F8, and F9a Facies caused by Low-Density Turbidity Current. Volcanogenic turbidite deposits in Halang and Kerek found in Bouma Sequence from Ta until Td. Sand and shale ratio of Halang formation is 1:1,14 with an average of sand thickness 20 cm sandstone, whereas Kerek Formation has 1:1,42 sand and shale ratio. The facies in Semilir Formation and Kebo-butak Formation is Smooth to Channeled – Channeled Portion of Suprafan Lobes on Middle Fan, with F2, F3, F4, F5, F8, and F9 Facies.

The reservoir potential of Halang and Semilir formation is rated fair because of the disturbance from volcanic components, whereas the potential reservoir of Kerek and Kebo-Butak is good. Kebo-Butak Formation came into the best reservoir quality with 780 centimeters maximum thickness and 50 centimeters minimum thickness. Kerek Formation also has a good reservoir quality because of the heterogeneity of the sand and shale ratio and widely spread along Kendeng Basin.

Introduction

Deep-water deposit along Central and East Java containing variety of texture, facies, and geometry that influenced by the materials of Oligo-Miocene Volcanic Arc in the center of Java Island that forming the volcanogenic turbidite deposits. There are four formation of volcanogenic turbidite deposits in the research area. There are two different location of the research area, Central Java as the location of Halang and Kerek Formation, and East Java as the location of Kebo-Butak Formation and Semilir Formation.

Reservoir potential in onshore Java is one of interesting discussion for long time, particularly reservoir potential in East Java Basin and Central Java Basin that has long been known as the area for hydrocarbon exploration. The sedimentary deposit in Central and East Java Basin commonly consist of deep-water deposit as the reservoir with various quality potential and geometry. Potential reservoir geometry depends on how, when, and the type of environment (Praptisih & Kamtono, 2011). Meanwhile the reservoir quality depends on the characteristics of the sedimentary rocks.

This research is to address the comparison of those four volcanogenic turbidite deposits formation for potential reservoir quality based on the outcrop data of those four formation. Those four formation contain thick sandstone as the potential reservoir

Regional Geology

Halang Formation Comprise of calcareous sandstones, pebbly sandstones, tuffaceous sandstone, marl, tuffaceous marl, claystone, claystone marl, breccia marl and calcarenite. Based on biostratigraphic data, the age of this formation is Miocene (N15-N18). Halang Formation were deposited in submarine fan on neritic depth. Sedimentary structure found in Halang formation are parallel lamination, graded bedding, load cast, and ripple mark.

N10 – N17 of Kerek Formation generally contain intercalation of claystone, marl, calcareous tuffaceous sandstone and tuffaceous sandstone with sedimentary structure such as gradded bedding structure.

Kebo-Butak Formation distributed in the northern part of Baturagung Mountain. Oligo-Miocene Kebo-Butak Formation comprise of well bedded sandstone, siltstone, claystone, shale, tuff and agglomerate in the lower part, whereas the upper part consist of alternating sandstone and claystone with intercalation of dacitic tuff.

Semilir Formation deposited in submarine fan on shallow-deep marine. This formation consist of andesitic-dacitic tuff, dacitic pumice breccia, intercalation of tuffaceous and conglomeratic sandstone, conglomeratic claystone and shale. Semilir formation characterized of deep-water sedimentary structure such as parallel lamination, graded bedding. Semilir Formation deposited on Early-Middle