Depositional and diagenetic history of limestones and dolostones of the Oligo‐Miocene Kujung Formation in the Northeast Java Basin, Indonesia

• Published in: The island arc Vol. 28; no. 6
• Main Authors: Syah, Muhlash Hada Firman, Kano, Akihiro, Iizuka, Tsuyoshi, Kakizaki, Yoshihiro
• Format: Journal Article
• Language: English; Japanese
• Published: Melbourne John Wiley & Sons Australia, Ltd 01.11.2019; Wiley Subscription Services, Inc
• Subjects: Calcite; carbonate diagenesis; carbonate sedimentology; Carbonates; Chemical analysis; Climate; Colour; Diagenesis; Dolomite; Dolomitization; Dolostone; Hydrocarbons; Isotope ratios; Isotopes; Limestone; Lithology; Microcodium; Miocene; North East Java Basin; Oligocene; Oligocene–Miocene; Permeability; Porosity; Ratios; Reservoirs; Sea level; Sea level changes; Seawater; Sediment; Sediments; Shoals; Strontium; Strontium 87; Strontium isotopes; Water analysis; East Java Indonesia; Indonesia
• ISSN: 1038-4871; 1440-1738
• DOI: 10.1111/iar.12326

Lithological and geochemical features of platformal carbonates record the signatures of the global climates and the regional environmental settings and also reconstruct the diagenetic history and porosity evolution, which are essential to evaluate the potential of hydrocarbon reservoirs. This study investigates the platformal carbonates of the Oligo‐Miocene Krunji Formation of North East Java Basin, which are potentially significant hydrocarbon reservoirs. The carbonate sequence in a 283 m thick section at Kranji in East Java is subdivided into three lithological units: limestone unit 1, dolostone unit 2, and limestone unit 3, in ascending order. The strontium‐isotope ratios of well‐preserved calcite samples indicate the depositional period from Chattian (late Oligocene) to Burdigalian (early Miocene), which is consistent with ages of the foraminifer assemblages. Unit 1 consists of low‐porosity limestone, in which two horizons of subaerial exposure are recognized by the occurrence of red‐colored matrix and lower δ13C values. Unit 2 consists of dolomitic rock and exhibits coarse‐grained calcitic grains and cross‐stratified structure. Considering that this unit has been subject to dolomitization, the sediment of unit 2 was initially permeable and was likely deposited in a shoal setting. The overlying unit 3 of Aquitanian–Burdigalian age is characterized by a highly granular texture. High porosity and uniformly low δ13C and δ18O values indicate that Unit 3 was subjected to more intense meteoric diagenesis than the Chattian unit 1. This was likely a consequence of the Antarctic ice‐sheet expansion during the Oligocene/Miocene transition, which amplified sea level change. The unit 2 dolomite has high δ13C and δ18O values and a high 87Sr/86Sr ratio which resulted from the reflux of seawater into permeable the sediment body in middle‐late Miocene (Burdigalian Tortonian) following the deposition of unit 3. The porosity and permeability of the Kujung Formation were initially controlled by sedimentological processes, but largely modified by later diagenetic processes. The Oligo‐Miocene carbonate sequence in North East Java records the signatures of the global climate and the regional environmental settings. Meteoric diagenesis and dolomitization controlled porosity evolution and potential for hydrocarbon reservoirs of this carbonate sequence. 原油貯留岩として有望な北東ジャワ盆地の新生界炭酸塩岩の岩相と地球化学的特徴は，この時代の気候変動を反映した堆積と続成史を記録している. ジャワ島東部のKranjiに露出するKujung層はChattian(後期漸新世)〜Burdigalian(前期中新世)に堆積したものであり，3つのユニットに分けられる. 下部の約30 mは空隙率が10%以下の緻密な浅海成石灰岩である. 厚さ約100mの中部は今回初めて認定された苦灰岩ユニットであり，高い酸素・炭素・ストロンチウム同位体はドロマイトが中期中新世以降に海水還流で形成したことを示す.上部の約110mは空隙率が10%以下の浅海成石灰岩であり，Microcodium に富み，酸素同位体比が低い値に集中していることから，強い淡水成続成作用を受けたことが示される.これは漸新世/中新世境界での南極氷床拡大に伴う海水準変動幅の増加を反映したものと考えられる.