Keyword: taphonomy, sequence stratigraphy, mollusc

Transcription

1 TAPHONOMIC STUDY OF MOLLUSCS TO CONSTRUCT SEQUENCE STRATIGRAPHY ARCHITECTURE IN TANJUNG-1, MEDAN-1, BUAYA-1, BIAWAK-1 WELL,SIHAPAS GROUP, TANJUNGMEDAN AREA, CENTRAL SUMATRA BASIN, RIAU Taufan Wiguna 1, Dody Suryadi 1, Yudha Risnandar S. 1 Aswan 1, Muhammad Fahmi 2, Satia Graha 2, and S.I. Qivayanti 2 1) Department of Geological Engineering, Faculty of Earth Science Technology, Institut Teknologi Bandung (ITB), Jl. Ganesha 10, Bandung, 40132, Indonesia addresses: taufanwiguna@rocketmail.com 2) Chevron Pacific Indonesia, Rumbai, Riau ABSTRACT The section of Tanjung-1, Medan-1, Buaya-1, and Biawak-1 Well, in Tanjungmedan Area, Central Sumatra Basin, Indonesia represents a climatic change marine depositional cycles and is composed of a series deepening upward sequences based on molluscs taphonomic analysis. The molluscs taphonomic analysis was used to interpret sequence architecture in the molluscs association of Sihapas Group (Menggala Fm., Bangko Fm., Bekasap Fm.). In terms of geological terminology, taphonomic analysis could be defined as the study of geological process happened after the death process of organisms and transformed into fossils (such as molluscs fossils). Molluscs in this study are dominated by the Gastropod and Pelecypod Class. Four type of shell concentrations could be grouped from the taphonomic analysis point of view; as Early TST (Transgressive Systems Tract), Late TST (Transgressive Systems Tract), Early HST (Highstand Systems Tract), Late HST (Highstand Systems Tract), and LST (Lowstand System Tract). The cycles suggest as a product of the third Milankovitch Cycle as the sixth order deposition. Keyword: taphonomy, sequence stratigraphy, mollusc INTRODUCTION With the increase of consumption of hydrocarbon for industry and the daily use, oil and gas producers all around the world are expected to increase their production. The production of hydrocarbon can be increase by two methods, first, to find new field of oil and gas by exploration, and second, to maximize the production on the existing field of oil. One of the method is sequence stratigraphy. Sequence stratigraphy can be used to correlate every part of the rocks from one well to another that formed in the same system and limited by unconformity sides or correlative side. Distribution of a probable reservoir can be determined from one well to others or from one area to other areas using sequence stratigraphy.

2 Some approaches have a lot of used to analyze sequence stratigraphy, based on lithofacies, mineralogy and foraminifer fossil or another fossil. In this research, the author would try to used a new method based on the taphonomy of mollusc fossil in core samples. Taphonomy is a study based on the appearance of shell beds (layers of rocks contain mollusc, found in field or core) to find out and interpret geological occurrence (e.g. sea level change, etc) which occur after those faunas dead and become fossils. The sequence stratigraphy architecture analysis based on mollusc taphonomy has been applied in several area such as: Maryland, USA (Kidwell, 1988), Punta Balena, Ecuador (Cantalamessa, et al., 2005), Omma Fm., Hokuriku, Japan (Kondo, et al. 1998), Sardinia, Italia (Botquelen, et al., 2006), and Nyalindung Formation, Sukabumi, Indonesia (Aswan, 2006); base on surface surface samples. GEOLOGICAL SETTING Sumatera Island occurred as a result of a subduction of 2 (two) micro continents at late Pre-Tertiary and now become part of the Sunda Shelf Plate. Oceanic crust of the India-Australia Plate is subducted along the Sunda Trench, forming the Sunda Volcanic Arc. Sumatera Island consists of 3 (three) major basins, namely: North Sumatera Basin, Central Sumatera Basin and South Sumatera Basin (Figure 1). The research area is located in the Central Sumatera Basin (Figure 2). Central Sumatera Basin is a back-arc basin that developed along the west and south side of Sunda Shelf, located in the south-west of Southeast Asia. This basin was occurred due to the subduction of Indian Ocean Plate that was subducted under the Eurasian Continental Plate in the Early Tertiary (Eocene Oligocene) and as a series of a half graben structure that was separated by horst block. This basin has an asymmetric shape northwest-southeast trending. The deepest part is located in southwest and shallow to northeast. Some parts of this halfgrabben are filled by non-marine clastic sediment and lake sediment (Eubank and Makki, 1981; in Heidrick and Aulia, 1993). Sihapas Group reasearched only 3 formation. They are Menggala Formation, Bangko Formation, and Bekasap Formation. BASIC THEORY Sequence Stratigraphy Sequence stratigraphy application need mechanism and processes that control sedimentation pattern in basin. Sedimentation patterns at non-marine or platform area are controlled by two main parametric, such as sediment influx and accommodation space change as place of sediment deposit. Sequence deposit is boundaries by unconformity and also correlative conformity with unconformity. That boundary is sequence boundary (SB) (Allen, 1987). Sequence deposit is also marked by Maximum Flooding Surface (MFS). Sequence Boundary (SB) and Maximum Flooding Surface (MFS) have pattern that can be perceived at log, core, outcrop, seismic section. Stratigraphy Surface identification and facies heaping analysis become bed of sequence stratigraphy analysis (Allen, 1987). The complete sequence deposit will form stratigraphy cyclic which can be divided become three stacking pattern deputizing

3 different phase from regression and transgression. (Posamentier and Vail, 1988). Stacking pattern is system tracts (Posamentier and Vail, 1988). The system tracts are Lowstand Systems Tract, Transgressive Systems Tract, and Highstand Systems Tract. The system tracts have facies sequence (Figure 3). Lowstand Systems Tract (LST), this systems tract is deposited when relative sea level fall at offlap break with relative sea level up next. Transgressive Systems Tract (TST), this systems tract is deposited when relative sea level fall at offlap break with relative sea level rise next. Highstand Systems Tract (HST), this systems tract is deposited when accelerating of sea level rise start down after top period (maximum flooding surface/ MFS), when acceleration of accommodation smaller than supply. The sea level down s speed in the beginning will cause formed aggradations geometry and then become progradation geometry. 1) Infaunal, its body is buried by sediment. Its orientation is posterior facing to down and usually stable. 2) Semi Infaunal, Its living position s orientation is vertical (posterior facing to down) but only some of body buried in sediment. 3) Reclining, its living position has floating effect horizontally on sediment. Generally, molluscs with this living position have plannar shell. Shell modification such as wide surface and thorns help this animals to float. A few note that one of from this specimen own the thorn, what emerge as additional adaptation for the reclining of in soft sediment. 4) Epifaunal, its living position is clinched at sediment or another object (sea Plants). Sediment deposit that contain of mollusc fossil usually has no idicator fossils such as foraminifera, nanoplankton and palinomorf. The mollusc is good indicator for basin side area. Mollusc Mollusc can be used as deposit environment indicator. Freshwater environment is distinguished by apparition of Brotia, Melanoides, Tarebia, Physa, Thiara, Unionidae. Fluvial environment is distinguished by Psiloinio, Brotia. Swampy and estuarin environment is distinguished by Batissa, Parahyotissa, Cerithidea. Living position of fossils (mollusc) to the basin basement (river, lake, sea) can be divided into (Figure 4. (a)gastropods (b)pelecypod): Mollusc Taphonomic Application for Sequence Startigraphy According to the theoritical, system tract can be divided by mollusc contain (such as micromollusc), there are : a) Transgressive Systems Tract (TST): can be divided become early TST and late TST. early TST deposit will associated by Sequence Boundary in the bottom and have marking: gravelly (a lot of fragment with gravel size), a lot of there are pickings of mollusc shell destroyed (breaks). This matter is caused by strong energy at the time of precipitation process happened,

4 generally a lot of there are concretion and a lot of there are bioturbations because precipitation process happened in environment which shallow relative and close to sea surface. Sediment of Late TST have marking: a lot of finding of adult fossil and intact of mollusc, where the fossil will meet by teaming as its consequence we will get a lot of couple of fossil of mollusc shell (Pelecypoda) precipitated nearby (articulated shell). This matter is caused at the time of precipitation process happened by the existing energy relative minimize the ( calm current relative) because existing sediment supply relative a few compared to available accommodation space hence existing mollusc can expand better adult till and precipitated intact relatively. b) Highstand Systems Tract (HST): can divided become early HST and late HST. early HST deposit is distinguished by: a lot of juvenile fossils mollusc, this cause when sedimentation process the energy become strong, sediment supply mounting to accommodation space. Because of that, mollusc cannot growth better to adult. Others that, shells condition has not perfect, sometimes only fragments. Early HST also shows carbon strict. Late HST deposit is distinguished by: interval between the layers contain mollusc and the layer doesn t contain mollusc (barren). This matter is caused when late HST, sea level will often experience of the restating of sea level rise and fall which not too big before really happened the falling quickly at LST. c) Lowstand Systems Tract (LST): this sediment generally seldom be met because always at the same time with the big erosion occurrence. If only met generally in the form of amalgamation big clastic, and because its deposit system with big energy hence will complicate there are fossil from certain fauna inclusive of mollusc fossil. METHODS The data are well cores and logs gamma ray. Tanjung-1 well s depth from 831 feet to 201 feet, Medan-1 well s depth from 1414 feet to 308 feet. Buaya-1 well s depth from 1408 feet to 484 feet, and Biawak-1 well s depth from 1125 feet to 200 feet. Technically, the activities to be done are analyze the data core provided and analyze every depth to find taphonomy symptom which explained above and compare with log gamma ray s sequence stratgraphy interpretation. The pictures each taken as sample are the most representative part. DATA INTERPRETATION Many authors conclude that taphonomic characteristic of system tracts elements are: Early TST: disarticulated and fragmented shells bed, chaotic, strongly bioturbated, concretions, gravelly and thanatocoenoese suggestions. Late TST: articulated, life position shells concentration, high density of epifaunal articulated shell, deeper environment shells association, starved sediments (thin layer). Early HST: juvenile and/ or individual articulated in life position shells, shell density and species diversity are rather low, shallower environment shells association suggestion, sometimes

5 fragmented shells and carbonaceous stricts. Late HST: multi-event concentration/ alternately, disarticulated shells, shallow environment shells association... continued on next page In this study, each sequence is bounded with 2 sequence boundary. Some taphonomic characteristics that signify each sequence stratigraphic elements as mentioned above that reveals from Tanjung-1, Medan-1, Buaya-1, Biawak-1 well cores are: Early TST: erosional boundary, gravely, floating grain, dominant big molluscs fragments, strongly bioturbated (Tanjung-1: cycle 2; Medan-1: cycle 1, 8, 9; Buaya-1: siklus 2, 3, 15). Late TST: articulated, life position shells concentration, high density of epifaunal articulated shell (Tanjung-1: cycle 2, 3, 5; Buaya-1: cycle 3, 12, 13; Biawak-1: cycle 6) Early HST: mollusc small fragment dominant, mollusc fragments smaller than Early TST (Tanjung-1: cycle 2 and 7; Medan-1: cycle 1, 2, 4, 8, 9; Buaya-1: cycle 1, 2, 5, 8, 13; Biawak-1: 1,2), carbonaceous stricts (Medan-1: cycle 2 and 3; Buaya-1: cycle 14; Biawak-1: cycle 6). Late HST: interlayer between layer contains shell fragments and layer does not contain shell fragments (Biawak-1: cycle 1 and 2). LST founded in Tanjung-1 well : cycle 3, Medan-1 well : cycle 3, Buaya-1 well: cycle 5 and 9, and Biawak-1 well: cycle 2, tebal >50 feet, amalgamation big clastics, shallower deposition, and higly bioturbated (Buaya-1: cycle 5; Biawak- 1: cycle 2).