GeologicaL Society of MalaYJIIl - Circum-Pacific Council for Entrgy and MineraL RuourceJ Tectonic Framework and Entrgy Ruouf'CeJ of the WeJtern Margin of the Pacific BMin November 27 - Dtcember 2, 1992, Kuala Lumpur, MalaYJUz Sequence stratigraphy of the Middle Miocene-Pliocene southern offshore Sandakan Basin, East Sabah RoBERT H.F. WONG PETRONAS Exploration Department P.O. Box 12444 50778 Kuala Lumpur Abstract: A sequence stratigraphic study was performed in the southern offshore Sandakan Basin with the aim of deriving a workable chronostratigraphic scheme and identifying plays, including stratigraphic traps. The data set includes 1800 km of high quality 1990 seismic and 8 wells. The study area can be divided into a structurally complex landward northern province and a relatively undisturbed basinward southern province. Three chronostratigraphic units were identified, each bounded by Type 1 third-order sequence boundaries. Each unit consists ofthird to fourth-order sequences which can be correlated from landward coastal plain facies to basinward bathyal facies. Unit 1 (Middle Miocene-early Upper Miocene) is characterised by moderate progradation, moderate aggradation; Unit 2 (middle Upper Miocene) by high progradation, low aggradation; and Unit 3 Oate Upper Miocene-Pliocene) by high aggradation, low progradation. The positions of the prominent shelf edges in Unit 1, Unit 2 and lower Unit 3 indicate a southeastward progradation from Middle to Upper Miocene. Pliocene progradation was probably eastwards as suggested by N-S growth faults in the northeastern part of the study area, and is interpreted to have occurred within a ramp setting, as prominent shelf edges are lacking. The study led to the recognition of two play-types in the southern province, both associated with lowstand systems tract sediments. The fIrst is a slope fan play in Unit 1. The second is a basin floor fan play in Unit 2. Comparison of the locally derived onlap chart and the global onlap chart of Haq et al. (1987) highlighted some differences. The main difference is major Upper Miocene progradation in the study area versus Upper Miocene aggradation in the global chart, testifying to the importance of the interplay of local tectonics and sedimentation in the Sandakan Basin. INTRODUCTION PREVIOUS LITHOSTRATIGRAPHIC SCHEME A sequence stratigraphic study of the Middle Miocene-Pliocene was conducted in the southern Figure 2 illustrates the previous litho­ offshore Sandakan Basin, East Sabah, Malaysia. stratigraphic scheme of the study area. This scheme The study area is located within the Malaysian closely follows the geology of the Dent Peninsula sector of the southern offshore Sandakan Basin located to the south of the study area. An example just north of the Dent Peninsula, as shown in Figure (Haile and Wong, 1964) of the previous 1. lithostratigraphic correlation between two wells is The study was carried out with the following exhibited in Figure 3. This rather coarse correlation objectives: is based on loosely constrained biostratigraphy and 1) To derive a chronostratigraphic scheme that poor seismic data. A better correlation based on an would improve upon the previous accurate chronostratigraphic scheme is crucial for lithostratigraphic scheme which is applicable successful petroleum exploration in the study area. only to a restricted area. This new, chronostratigraphic correlation was 2) To identify play-types including stratigraphic required, to enable more precise mapping of traps such as basin floor fans and slope fans. reservoir, seal and source. Geol. Soc. MalaYJia, Bulletin 33, November 1993,. pp. 129-142 130 ROBERT H.F. WONG PENINSULA o 200KM '-----.. .......' LETOUZEY ET AL, 1 LEGEND ~ SEDIMENTARY BASINS _ STUDYAREA -v- VOLCANIC ARC ~ BASEMENT COMPLEX --200- WATER DEPTH IN METRES AXIS OF STRUCTURAL ~ (CONTINENTAL CRUST) GRAIN ~ NEOGENE VOLCANIC ROCKS _ _ SUBDucnON ZONE "V (PRESENT ACTIVITY UNCERTAIN) Figure 1. Sabah-Sulu Sea area, tectonic framework. G R PETROLEUI AGE STRATIGRAPHY 0 FORIATION GEOLOGY TECTONICS u II W E P He RES ". • ••••••• L PLEISTOCENE •••••••• .7 2.0 .--:-....!.-:.:~~.;f- TOGOPI I--COMPRESSIONI PLIOCENE L 0 A WRENCHING ,1:. • &.1 ..... .. E !,-COMPRESSIONI .. .. N ----· ..... GANDUMAN WRENCHING I U . !'{Y·.!tL. T -¢- ~ ~ .'7 EROSION ':-: -. -:-~ -......:..~. I-- -- i-GROWTH · ........... SEBAHAT ~ FAULTING I- ----=--~--- ~ • t-UPLIFTITlLTINGI M · ........ T _-- A * ~, EROSION MIOCENE I';;' 06.0 '.-:-.~~ S B TUNGKU ~ A • ~C~=;..: E N .. - o A 0 A 060· ":"y.:!:,-y ._ A LlBONG G K -UPLIFTI A L ~ "rJ "'- EROSION fo~ VYVyVV M A AVER 24.8 --~----:::r -;:;;..- ...... K · ..... -=..-=--=..- - , ~'7 U ............. .:..-=------ N LABANG ......••• 0 _____---- A · .. B .~: A OLIGOCENE .. : .---:-?"-=-==-- T i--'?---?- ==---=---=-=- A N G KULAPIS L ~~-.. ~~~~ A N ~----~-~ I ".,.,1-1- v v V v v c II CROCKER I 0 c CHERT EOCENE ~ K SPILLITE E · ~ + + + + II Figure 2. Lithostratigraphic scheme. SEQUENCE STRATIGRAPHY OF THE MIDDLE MIOCENE-PLIOCENE SOUTHERN OFFSHORE SANDAKAN BASIN, EAST SABAH 131 DATA AVAILABILITY and comparing it with the global onlap chart of Haq et al. (1987). The data available for this study include seismic, Figure 6 indicates the location of four of the well and outcrop data. The seismic data comprised 1990 seismic lines used in the study. Sequence 1800 km of high quality 1990 vintage with an stratigraphic interpretation on segments of these average grid size of 3 km x 3 km, and limited lines will be discussed. reprocessed 1968, 1971 and 1973 vintages. The location of the 1990 seismic grid is shown in Figure 4. INTERPRETATION EXAMPLE The well data comprised eight wells with GR and sonic logs, a biostratigraphic review of ditch Figures 7 and 8 show Line No.1, Part 1 cuttings from five of the wells and lithofacies (northwest segment) and Part 2 (southeast segment) descriptions of 110 m of cores from selected wells. respectively. Figures 9 and 10 illustrate the The biostratigraphic review included foraminifera, interpreted versions of these segments, showing calcareous nannoplankton and quantitative three chronostratigraphic units, each bounded by palynological analyses which provided both age Type-l third-order sequence boundaries. When zonation and depositional environment combined, these three units make up a second­ interpretation. No synthetic seismograms were order event with an age range of about 17 Ma. available, hence, time-depth curves were used to Each unit comprises one or more third-order tie wells to the seismic data. Such ties are believed sequences. A sequence boundary within each unit to be sufficiently accurate for the study purpose. can be readily recognised seismically by a strong The location of the eight wells is also shown in flat event on the shelf changing abruptly with a Figure 4. pronounced shelf edge (circled) to an oblique clinoform and then flattening again towards the STRUCTURAL SETTING basin plain. The location of well No.6 which bottomed in Middle Miocene outer neritic shales is The study area can be divided into northern also shown on this line. and southern structural provinces, which are Various seismic facies can also be observed on separated by the northeast-southwest Segama­ Line No. 1. They include hummocky, discontinuous Pegasus Ridge (Fig 5). The sediments in the reflectors (in Unit 2 and lower Unit 3); parallel, northern province were deposited in a more continuous reflectors (in all three Units); mounded landward environment and have undergone various with internal parallel reflectors (in upper Unit 3); phases of deformation from Middle Miocene to oblique-shingled clinoforms (in Unit 2); sigmoidal­ Pleistocene. The western part of this province is oblique clinoforms with associated chaotic reflectors characterised by northeast-southwest wrench­ (in Unit 1 and lower Unit 3); and parallel to gently related flower structures whereas the eastern part mounded reflectors at toes of clinoforms (mainly in is dominated by growth faults. Units 1 and 2). Seismic reflection terminations In contrast to this, the basinward southern include onlaps in Unit 1 and lower Unit 3 and province encompasses a zone of relatively toplaps in Unit 2. undisturbed sediments. Subsidence and sedimentation with little or no structuring since SEQUENCE BOUNDARY MAPPING the Middle Miocene characterises this province. The interpreted sequence boundaries were INTERPRETATION PROCEDURE correlated and mapped and the pronounced shelf edges indicated. The intervening sequences are The study involved sequence boundary mapping designated Unit I-Seq. A-E; Unit 2-Seq. A-C; and (Mitchum et al., 1985) using seismic data tied to Unit 3-Seq. A. Younger sequences of Unit 3 (Unit well data, seismic facies analysis (Mitchum et al., 3-Seq. B-E) which are lacking in pronounced shelf­ 1977, part 6) and systems tract identification from edges, are identified on well-logs by recognition of well logs (Van Wagoner et al., 1990). Depositional valley incisions. models were derived once the various sequences Figure 11 indicates the seismically-deflned and their component systems tracts were deflned. Middle-Upper Miocene shelf edges. Overall Consequently, prospects and leads which included progradation is towards the southeast. Pliocene stratigraphic traps of slope fans and basin floor progradation is interpreted to be towards the east fans were identified from the depositional models. as suggested by N-S growth faults in the Finally, more precise dating of the sequence northeastern part of the study area and is boundaries could then be