oa Stratigraphic Architecture and Process Variability of the Transgressive Paralic Cycle II (Early Miocene) Interval, D35 Field, Balingian Province, Offshore Sarawak

Detailed and integrated core analysis (physical sedimentology, ichnology and biostratigraphy) and extensive well log calibration and correlation of over 70 wells (exploration & sidetrack), constrained by seismically-defined surfaces, reveal an intricate stratigraphic architecture of the Cycle II (Early Miocene) section of the D35 Field, constructed from coastal and marginal-marine depositional intervals. The overall architecture reflects a complex interplay and variability of sediment supply, relative sea level changes and depositional processes. Three major stratigraphic intervals are defined by lithofacies characteristics and fieldwide stratigraphic surfaces. The lowermost Interval I is defined by a field-wide basal erosional surface which partly truncates a correlatable coal-capped paleosol horizon in places, and overlies a prominent condensed section. The erosional surface is interpreted as a sequence boundary; it marks a significant fall of relative sea level across the field. The surface is overlain by a thick interval (> 100 ft) of sand-rich progradational unit; it is dominated by cross-bedded sandstones and pebbly-sandstones, which are interpreted to represent a coastal mouthbar complex, which in places are channelised. These cross-bedded sandbodies are interbedded in places with brackish, bioturbated (with several Glossifungites intervals) and wavy-to-irregularly laminated tide-influenced sandstones, indicating that the mouthbar complex is in contact with a brackish water body, and is affected by intermittent marine flooding. This interval is the distal, Lowstand System Tract. A major flooding surface, correlatable across the field, separates the Lowstand Interval I from the overlying Early-to-Middle Transgressive Interval II. This comprises aggradational-to-retrogradational units of laminated-to-massive, brackish estuarine mudstones, interbedded with tidal flat and minor bay head delta sandstones, often capped with minor coals and paleosol horizons. The stratigraphic and cyclic stacking of brackish mudstones-paleosol-coal facies association signals recurring marine flooding and land progradation throughout the deposition of Interval II, with minimal sand input. The Late Transgressive Interval III is characteristically marked by the dissappearence of coal-capped parasequences, and the domination of thick brackish estuarine mudstone. It is distinguish from the E-TST in many wells by the last coal before thick mudstone interval. A Maximum Flooding Surface separates the L-TST from the Highstand System Tract (HST). This is marked by the return of the coastal and estuarine margin, coal-capped parasequence set. The HST here is thin and poorly developed, or has been eroded by the subsequent fall in relative sea level, which forms the upper bounding surface for Cycle II and the Sequence Boundary for Cycle III. This truncation surface marks the end Cycle II.