In part 1 I hope I summarised the inevitability of major changes to regional geology around SE Asia based on the deliberate search for tectono-stratigraphic anomalies, and the recognition of new patterns in those anomalies. First, each anomaly was verified from original data, quantified by techniques such as geohistory analysis, and compared to adjacent wells or outcrops. This is because stratigraphic features, including unconformities, have properties that are strictly constrained by Walther’s Law.
A new paradigm implies a significant shift in perspective and ideas. Is this the case for SE Asian geology?
Well, changes to regional geology will not impact exploitation of local fields or clusters, but it will impact where explorers go to look for high value-added exploration. How much change comes from a new understanding of areas that were previously hidden from us, or downgraded by over-simplified models (the old paradigm). For example, no one explored onshore Java for reefs as the old paradigm knew such reefs stopped at a shelf edge to the north, and under the island of Java there was a deep, palaeo trough. This all changed in the 1990s and reefs were found including many large oil and gas fields. Part of the new geological paradigm included Neogene inversion of old lows so the new suite of reefs were usually located in modern synclines, and hard to image on seismic. This was such a major change in tectono-stratigraphy, which separated eastern Java depocentres under the Java Sea from those under Java and the Madura Straits. The very concept of a single “East Java Basin” should have been invalidated, but it has persisted, linked to the old paradigm, which will not shift until it is replaced. There are many other areas around SE Asia where this kind of paradigm shift needs to be carried out and entire new petroleum systems considered. Note that the largest oil and gas fields in eastern Java, with over a century of hydrocarbon productions, are all in the area that required the new paradigm. This is also the case in North Sumatra.
So what patterns are currently emerging from the new paradigm shift? First, if you want to understand the old paradigm I recommend Doust and Noble (2008; see this link). The new paradigm requires close examination of the history of facies, unconformities, subsidence and uplift events, especially rates of change, between adjacent basins but also regionally. The anomalous data, such as described in part 1, points to many times of coeval tectonic change around the region. Initially episodic subsidence, but later involving abrupt re-configuration of basin architecture during the slower process of Miocene to Recent compression. This defines regional sequence stratigraphy and natural genetic units of sedimentation that can be correlated between basins. This is important as the summary of Doust and Noble, and many others (I am not picking on Doust and Noble, but theirs is the best presented summary of widely held beliefs), simply do not look at many areas with thousands of meters of older Neogene or Palaeogene sediment (often with oil seeps; West Java, Mangkalihat Peninsula, Sabah, North Sarawak). These need a regional first-order sequence stratigraphic framework in order to begin evaluating areas of overlooked potential. As the giant onshore Java fields were found in the 1990s they were added to Figure 11 of Doust and Noble, but the text and paradigm did not change. This is not useful science, it is a technical history.
During the episodic extensional phase (c 45 to 15 Ma) it was overlooked that there were two axes of extension, the well known South China Sea, but also the Makassar Straits. As discussed in a recent paper reviewed here and again here, there were a series of reefs in the Makassar Straits that became extinct at the Oligo-Miocene boundary (see map below; the double circle symbol). The Sultan-1 well is a good example of the simple power of geohistory analysis as the good data from this well, which drilled to a volcanic basement, shows the magnitude of the accelerated subsidence that affected a wide area at a time (top J70) when the old paradigm (Doust and Noble, 2008) thought that faster rift tectonics were passing into slower thermal sag or post-rift tectonics.
At exactly the same time as this accelerated rifting in the Makassar Straits there was ridge jump and accelerated rifting (including rift to drift transition) in the South China Sea (see maps below). The axis of the new ridge jump spreading in the SCS was roughly parallel to the axis of the subsidence (without sea-floor spreading) under the Makassar Straits.
This is just one example of the new paradigm but it is clear and profound. Suddenly we have a single tectonic model that links most of Sundaland (this was also the time of onset of deposition of the Arun reef in North Sumatra). It reveals a whole new history of basins free of inappropriate concepts like “back-arc”, or rift to sag architecture.
The complete new paradigm consists of a framework of empirical data for Eocene to Pliocene sediments, through extensional and compressional phases. It has a secondary part of trying to fit this detailed empirical data into a new geological, especially plate tectonic, framework.
This is perhaps the first evidence-driven review of the geology of SE Asia since van Bemmelen (1949). The exciting part is that we can use the same analytical methods that are used to build the new paradigm to extrapolate processes into un-drilled areas, or slightly older sections. The same approach will propose and test new hypotheses on regional geology and tectonics in an excellent natural laboratory.
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