Sandy mass transport deposit model and its natural gas accumulation in the Yinggehai Basin

The sandy mass transport deposit of the first member of the Huangliu Formation in the Upper Miocene was newly recognized using 3D seismic data and drilling data collected from the XF13 area of the Yinggehai Basin. The depositional model and natural gas accumulation mechanism have been investigated. Cores recorded sedimentary characteristics of sandy mass transport deposit very well, developing massive stratifications and parallel stratifications, abrupt contact surfaces, mudstone avulsions, and deformation layers. The sandy mass transport deposit had distinct seismic reflection features compared with those of surrounding rocks. They consisted of multiple periods of sedimentation, and each period experienced early erosion, filling, migration and a late abandonment stage. They developed a double-layer sedimentary architecture, which consisted of massive thick compound lobe sandbody and sheety sandbody, respectively. Each period of sandy mass transport deposit developed sandy debris flow massive thick sandstones with slide-slump movement near provenance, and muddy turbidity current deposits laterally far from the source. A new model with a transgressive-highstand systems tract and outer continent environment was proposed. A case study was made in the XF13 gas field. Through the integrated analyses of sandy mass transport deposit, structural-lithological trap formation, fault/fissure migration pathway and natural gas pooling process, natural gas accumulation mechanism were considered. The central and deep sections of the basin have favorable geologic conditions for gas enrichment and preservation.