A Successful Subsalt Exploration Example of M Block in The Eastern Margin of Precaspian Basin

M block in eastern margin of Precaspian Basin is the first CNPC ’s risk exploration project in Central Asia & Russia, which has been prospected by many companies and no discovery was found. The main exploration difficulties are subsalt trap recognition’s trouble caused by salt domes with giant thickness (greater than 1000m) which lead to subsalt target stratum’s pull-up and false faults in the Permian and strong heterogeneous reservoir prediction in carbonate reservoir buried near 4000m. Technology was developed based on the difficulties, in the aspect of subsalt structural recognition, fine interpretation by multidisplay methods, salt domes’ margin identification using boundary detecting technique, time-depth conversion methods of raytracing procedure, forward modeling examining velocity field’s precision, vertical seismic profiling (VSP) detecting local structural shape, palaeotectonic restoration analyzing trap’s origin, prestack depth migration verifying trap’s validation, salt dome’s influence was wiped out continuously and strata’s real shape was resumed which leads to the increase of drilling success ratio. In the aspect of carbonate reservoir prediction, favorable reservoir zones in Carboniferous strata were outlined based on the study of logging assessment, sequence stratigraphy, sedimentary facies, seismic inversion, fracture and petroleum detection, attribute analysis. At the same time, REI(reservoir evaluation index) and CI(cave index) indexes were initiated, and carbonate reservoir’s logging assessment standard in this block was established that improves reservoir prediction’s precision. In the aspect of comprehensive evaluation, 3 structural accumulating belts were divided after the study of reservoir types, which are west structural belt, central structural belt and east structural belt. The different belts’ play was evaluated synthetically. The main and potential play was pointed out in different belts. The regular patterns about subsalt carbonate accumulating were generalized. Middle ramp region should be break through first is our bulk exploration deployment clue, and a big field with more than a hundred million tons reserves was discovered which is the biggest subaerial discovery since the Kazakhstan being self-governed and guarantees the abundant supply for the Central Asian pipeline.