SURFACE GEOCHEMICAL EXPLORATION AND HEAT FLOW SURVEYS IN FIFTEEN (15) FRONTIER INDONESIAN BASINS

摘要

TDI-Brooks International, Inc., under contract to TGS-NOPEC, conducted an extensive Surface Geochemical Exploration (SGE) survey in 15 frontier basins in Indonesia between April 2007 and March 2008. The Indonesian Frontier Basins (IFB) Project is a multidisciplinary program designed to generate meaningful petroleum exploration data throughout Indonesia's under-explored frontier basins. The SGE program was the final step in this study designed to evaluate hydrocarbon charge in these offshore frontier basins. Core sites were selected based on multibeambathymetry/backscatter anomalies at sites potentially indicative of seepage combined with 2-D seismic data where available to assist in delineating seep conduits from depth to the seafloor. A 1-km long Chirp subbottom profiler line was acquired over each target site to further refine the core position. Sub-bottom profiles were used to adjust some targets toward sites of potential seepage, or used to avoid hardgrounds that would yield little to no recovery as well as damage the core. Approximately 1,400 piston cores were acquired during the study using the TDI-Brooks research vessel R/V GeoExplorer. The 1,000 kg piston core rigs used 3, 6 and 9 m long × 7.5 cm OD barrels, and were positioned with an Ultra-Short Baseline (USBL) navigation system to within 25 m of the pre-selected seabed targets. Core recovery was typically 3 to 8 m of sediment column, although recovery was affected by bottom type. Many sites with carbonate hard-bottom were selected during this program, and core recovery at such sites was sometimes limited to less than 1 m. Three sediment sections per piston core were typically sampled at about 1 m intervals from the bottom of each core todetermine the presence of any thermally sourced hydrocarbons.Analytical screening measurements included: (1) the fluorescence spectra from bagged sediment sections using dried-sediment solvent extraction, (2) the C_(15+) hydrocarbon concentrations by gas chromatography in the same sediment extracts, and (3) the interstitial light hydrocarbon gas concentrations from separately canned sediment sections using wet-sediment gas partitioning and gas chromatography. From these analyses, screening indicators of migrated liquid and gaseous thermogenic hydrocarbons were developed and used to qualify cores and distinguish them from defined background thresholds for this study area.Sediment extracts of this study, each representing one section of a core acquired at a site of interest, were qualified for the unambiguous presence of migrated liquid petroleum (thermogenic) hydrocarbons by the following criteria using background thresholds of: TSF Maximum Intensity ≥ 10,000 and UCM ≥ 10 μg/g, or T/D ≥ 1.5. Using these screening criteria, 118 of the 927 cores (13 percent) exhibited unambiguous evidence of liquid petroleum hydrocarbons. This is an unusually high percentage of oil seep shows for our SGE surveys (compared to about 8 percent on average globally, and <5 percent on average in frontier basins), and attests to the potential of the region as well as to the quality of the targets selected.The concentrations of alkane gases in the canned samples typically increased with depth in a core. The strength of the interstitial gas screening indicators as prospecting tools improved significantly with depth over the 3-8 m range typically retrieved by our method of piston coring. Coring carbonate hard-bottom sites reduced such depth-recovery at many of the sites, but high concentrations of gas were measured at some of these hard-bottom sites. Sediment samples canned for interstitial gas measurements, each representing one section of a cpre acquired at a site of interest, were qualified for the unambiguous presence of migrated thermogenic gases in the following manner: (1) an ethane/ethene ratio of at least unity, and (2) the product of the ethane/ethene ratio and the total alkane gas concentration (in ppmV) being at least 100, in at l