Di Seputaran Delta Mahakam Tidak Semua Reservoir Batu Pasir Harus Fluvial atau Deltaic, Ada Juga yang “Marine”
Dirilis pertama di Facebook pribadi.
Yang dimaksud dengan pengertian “marine” di sini adalah batu pasir yang dalam proses transportasi dan pengendapannya lebih dipengaruhi oleh proses gelombang laut dan atau pasang-surut daripada proses influx sungai memasuki laut. Tercakup juga dalam pengertian itu, provenance akhir dari batu pasir tersebut berasal dari endapan pantai yang sudah ada sebelumnya dan/atau endapan dataran delta bawah/muka delta (LDP-DF) dari proses delta sebelumnya. Karakter reservoir-nya lebih homogen, pelamparannya lebih “sheet-like”, dan kemungkinan por-perm-nya juga lebih bagus daripada reservoir fluvial-deltaic yang mengalami proses diagenesa/burial yang setara.
Contoh di lingkungan modern dapat diamati di lobe utara dari Delta Mahakam modern, juga di “spit-bar” Marangkayu, dan juga pantai panjang berpasir di daerah Manggar sampai Handil. Untuk daerah utara, satu paper penelitian aktif telah dipresentasikan dalam AAPG Hedberg Conference on “Variations in Fluvial-Deltaic and Coastal Reservoirs Deposited in Tropical Environments”, di Jakarta 29 April – 2 Mei 2009 yang lalu (lihat abstract terlampir). Paper yang sama, dengan tambahan data baru, juga diminta (invited) untuk dipresentasikan di AAPG Lousiana April 2010 mendatang di sesi khusus Hedberg Conference Papers. Komponen morfologi/sub-environment-nya diantaranya adalah: eroding shoreline/beach, spit bar - brackish lagoon, transgressive barrier bar, wave dominated delta, flaring tidal channel mouth, dan northernmost lobe of main Mahakam delta.
Contoh di singkapan dapat dilihat di tebing belakang warehouse Samarinda (tengah), singkapan Sepaku KM 18-21 (selatan), dan di sepanjang punggungan Semberah (utara).
Penerapan konsep analog ini dalam pemetaan bawah permukaan memungkinkan untuk jadi alternatif pemecahan masalah kemenerusan dan/atau kompartementalisasi reservoir di lapangan-lapangan migas seputaran Delta Mahakam.
Selamat menikmati.
AAPG Hedberg Conference:
“Variations in Fluvial-Deltaic and Coastal Reservoirs Deposited in Tropical Environments”
April 29th - May 2nd, 2009
Jakarta, Indonesia
Marine processes in the northern lobe of Mahakam Delta and Marangkayu Spit Bar:
Implication on paleogeography model in the subsurface.
Andang Bachtiar — Exploration Think Tank Indonesia
Armein Sulaiman — Geosains Delta Andalan
The morphological components of northern lobe of Mahakam Delta combined with adjacent shore-line of Muara Badak and Marangkayu Spit Bar to the north of the delta is being investigated to reveal the interplay between fluvial and marine processes in their making. These specific areas are hypothetically considered as analogues to the paleo-geography of the reservoir sections in the subsurface of Semberah Field, which is located 15-20 KM inland to the west and parallel to the shoreline. Bathymetric survey, bottom grab sampling, and shallow sediment coring are conducted in 4 locations representing upper delta plain, lower delta plain, upper shore-face and fore-shore. Surface sediment observations are also performed in the spit bar area.
The present-day current direction measurement and the grain-size distribution along the shorelines suggest at least two terrestrial sources of sediment provenance filling in the area. In the northern lobe of Mahakam Delta the tidal effect is more pronounced than the wave, while in the Marangkayu Spit Bar the wave action is the most dominant one. In both areas, fluvial processes can still be seen acted on certain sub-environments dominantly. Brackish water, clay drapes, flaser and wavy ripple lamination and suspension feeder burrows characterizing the delta plains of the northern lobe of Mahakam Delta. Saline sea water, trough cross bedding, low angle parallel bedding and sand burrows are typically observed in the Marangkayu Spit Bar area.
Analogizing the present-day shoreline of Northern Lobe of Mahakam Delta – Marangkayu Spit Bar to the Miocene subsurface environment in oil and gas field adjacent to the area, we found a very pronounced one-to-one matching, which needs further works on mapping and comparing the sand body geometry between the two realms.
