Prediction Of Deep Marine Carbonate Reservoirs In Gaoshiti-moxi Area,sichuan Basin

In Sichuan Basin,the major gas-producing area in China,the Leshan-Longnvsi paleo-uplift has long been considered by geologists as a favorable belt for the accumulation of deep Sinian-Lower Paleozoic oil and gas.Through continuous research and exploration,190×10~4 m~3/d of gas was obtained from the Longwangmiao Formation in M8 well after well testing.However,this formation in the subsequently drilled G2 well was only 4.5 m in thickness and no industrial gas flow was obtained,proving that the Longwangmiao reservoirs are heterogeneous and their prediction is very difficult.The main problems faced by exploration of Longwangmiao Formation are how to define a high-quality reservoir development zone and determine some drilling targets,including(1)this formation is generally buried in a deeper depth ranging from 4400 m to 4900 m,and complex seismic reflection to the top Longwangmiao Formation and fast lateral change may result in difficultly tracking this top and many difficulties in fine interpretation;(2)extremely strong heterogeneity in vertical and horizontal direction may bring about complex seismic response pattern to difficultly develop a unique reservoir response;(3)for this formation,not only large variation in thickness but also great lateral change of petrophysical property cause that grain beach reservoirs with thin thickness are difficult to forecast;(4)the relationship between gas and water and their plane distribution law are unpredictable.These problems restrict the process of exploration and development of deep carbonate reservoirs in Gaoshiti-Moxi area.To break through these bottlenecks in technology is of great practical significance for the exploration and development of deep carbonate reservoirs.Taking carbonate reservoirs of the Lower Paleozoic Longwangmiao Formation in Gaoshiti-Moxi area as examples,this study carries out fine processing and interpretation of seismic data,explores reservoir prediction methods and establishes a set of seismic approaches to predict Cambrian reservoirs based on reservoir seismic response model,finally proposes some favorable well locations on the basis of the main characteristics and distribution of these reservoirs,all providing technical support for the exploration and development as well as reserves declaration of the Lower Paleozoic Cambrian gas reservoirs in Sichuan Basin.In addition,our study solves the key issues,such as fine interpretation of deep and low-amplitude structures,reservoir prediction,and oil and gas detection,furthermore develops some key technologies to forecast deep marine carbonate reservoirs in Sichuan Basin.The main achievements are as follows:(1)Structures in Longwangmiao Formation of Gaoshiti-Moxi area and contact relation among them are made clear according to fine interpretation of both horizons and faults by high-precision 3D seismic data.(2)Seismic response features of Longwangmiao reservoirs are established by geological forward modeling,such as“bright spot”reflection inside representing a seismic response to the reservoirs with relatively high permeability and porosity,or stronger wave peak amplitude(“bright spot”)indicating dissolution vugs and pores more developed in the reservoirs.(3)Longwangmiao reservoirs are qualitatively and quantitatively predicted by means of methodological test to optimize seismic attributes and a technology of prestack geostatistical inversion.It is considered that the reservoirs in Moxi area are featured by wide northeast-striking distribution and large area whereas the Longwangmiao reservoirs poorly developed in adjacent Gaoshiti area mostly present a stripped distribution.(4)It is considered after petrophysical analysis that,to divide a gas reservoir,a non-reservoir,or a water layer can not depend on a single petrophysical parameter;moreover,a crossplot of both??and P-wave impedance can be well used to distinguish the favorable gas reservoir.After petrophysics-based oil and gas detection,it is made clear that in Moxi area,there are 9 favorable gas-bearing zones of Longwangmiao Formation with a total area of 462.3 km~2.