A review of the Sarawak Cycles: History and modern application Peter Lunt 1* & MazLan Madon 2 1 Petronas Carigali 2 Petronas, Malaysia Petroleum Management Level 14 Tower 2, Petronas Twin Towers, Kuala Lumpur 50088, Malaysia *Corresponding author: Peterjustin.lunt@petronas.com.my Abstract: This review examines the history of the Sarawak “Cycles” and their application in subdividing offshore Cenozoic stratigraphy. The Cycles are widely but inconsistently used, at least in part because important reports and data collections were never published (e.g. Geiger, 1964; Hageman et al., 1987; Hageman maps, 1985, eventually reproduced in Madon, 1999 and Hutchison, 2005; and Taylor et al., 1997) or were published in highly abridged summaries only (Ho Kiam Fui, 1978; Hageman, 1987; Mansor et al., 1999). The lack of a data-audit trail left open possibilities for ambiguity and confusion, as has been commented on by several workers (e.g. Snedden et al., 1995; Ismail & Tucker, 1999). This account reviews the major contributions, published and unpublished, and the concepts of the Cycles, especially over times of geological change. The data behind the model is cited in order to give conidence when integrating Sarawak stratigraphic data into a regional geological model. The Cycles began with an assumption that transgressions over regressive surfaces were distinct and approximately synchronous events for correlation. By including biostratigraphic data these transgressive events could be traced into clay-dominated areas, where lithological and seismic contrast was weak. This integrated approach was carried out through the 1970’s and 80’s, during which time the data pushed the model towards a three dimensional view of sedimentation, with the basin shape evolving and changing through time. However, development of a full tectono-stratigraphic model paused during the period of accelerated seismic capabilities of the 1990’s and early 21 st Century. While geophysical data coverage increased, application of geological analyses decreased, and the integrated approach lost momentum. This review aims to re-establish the role of the Cycles as a part of a large scale geological model. An initial integration with regional geological events is attempted, linking some of the Cycle boundaries with times of known tectonic change. Keywords: Sarawak, stratigraphy, Cycles, history, application INTRODUCTION A method was required to sub-divide and correlate well sections in the thick deltaic sediments deposited offshore Sarawak. Subdivision would allow different facies to be to be mapped and their development traced through time, while correlation would allow time equivalence to be demonstrated independent of facies. However correlation was made hard in a section where age diagnostic fossils are so scarce that evolution and extinction datums can rarely be observed with any conidence. Work on this problem began before high quality seismic and sequence stratigraphy were available. The report of Doust et al. (1977) shows how the earliest seismic improved up to the mid 1970s but, even with modern data, the task of proving correlation and building an integrated stratigraphy remains dificult. Early workers recognised about eight phases of sedimentation they called “Cycles”. Understanding how these Cycles were recognised and how they it a regional model remains important. The irst stratigraphic model based on Cycles led to the identiication of geographic areas, each with its own distinct stratigraphy. The Central Luconia Province, named after the Luconia Shoals where modern reefs are exposed between the locations of the G2 and G10 wells (Figure 1), is the area where reefal carbonates were seen on seismic and drilled with commercial success. The Balingian Province, named after a coastal town, is the area south of where the reefs pinch-out, characterised by mixed luvial to shallow marine sands and clays, and only very rare, thin limestones, deposited from Oligocene to Recent times. Seismic also showed that just west of Balingian town there was a major reduction in basement depth over a step-like feature called the West Balingian Line and much thinner clastic strata were drilled over the Tatau Province. Northwest of the Tatau Province, and west of Central Luconia, the Neogene sediments thickened considerable and this area was named the West Luconia Province. The North Luconia province is Figure 1: Location map. 0126–6187 / © 2017 Geological Society of Malaysia. All rights reserved. Bulletin of the Geological Society of Malaysia, Volume 63, June 2017, pp. 77 – 101