Reservoir Architecture Analysis of Braided-River Delta Front in Lunnan-2 Oilfield, Tarim Basin, Western China

Located in the middle of Tabei uplift in Tarim basin, Lunnan-2 oilfield has been in production since 1992 with the extraction ratio of 38.58% and the water cut 92.59% till now. Production has been hampered by the economic... [ view full abstract ]

Located in the middle of Tabei uplift in Tarim basin, Lunnan-2 oilfield has been in production since 1992 with the extraction ratio of 38.58% and the water cut 92.59% till now. Production has been hampered by the economic risks related to the uncertainties in the size, spatial distribution and mutual contact of sand-bodies, caused by the continuous migration of channels, vertically-stacked and laterally-amalgamated sediments. The study aims to put forward a comprehensive analysis method for reservoir architecture of braided-river delta front. It focuses on distributary channel bodies using data from Huangshanjie outcrops in the north of Kuqa County, and analogues in mature oilfield. The architecture elements are identified as: distributary channel, mouth-bar and tributary bay. Their vertical superimposition and lateral juxtaposition exhibit the sand-on-sand contact near their apex, leading to interconnected and huge volumes. From wireline-log interpretations three factors can be employed hereafter to judge whether the detected sand-bodies among adjacent wells actually belong to the same sand-body: 1) different elevation of single sand-body to the formation interface (or marker bed), 2) fine grain-size sediments between sand-bodies, 3) different shape of well logging curves. Results show that single mouth-bar has the length up to 2400 meters, width up to 1450 meters and thickness ranging from 3.5 to 7 meters. A statistical and regressive analysis of the mouth-bars’ size establishes the mathematical relationship with a high correlation coefficient of 0.7. An application of strata-thickness ichnography, RMS amplitude and ants-body attribute strata slices derived from seismic attribute-volume contributes to identifying the boundary of sand-bodies. The validation is proved by taking advantage of the production-absorption capacity section, effective tracer, production curves, water-drive front and other dynamic analysis. With the analytical results above, a great significance to 3D reservoir geological modeling is shown, the work will help establish a predictive model for such reservoirs in the Triassic subsurface of Lunnan-2 oilfield.