Fluid Prediction Methods For Complex Carbonate Reservoir Based On Pre-stack Inversion

At the period of Ordovician system,carbonate is broadly developed on A area at Tarim basin.Influenced by weather denudation and water solution,it turns into secondary carbonate and its dominant reservoir spaces are dissolved caves.For this kind reservoir,it shows obvious‘bead-like’response on the seismic section.Affected by structures,these dissolved carves are filled with different fluid or mud,while we can’t discriminate these fillings only by post-stack seismic data.Therefore,pre-stack inversion plays an important role in fluid prediction and we can assign the subsequent drilling wells according to the inversion results.Firstly,we do pre-stack inversion in time domain using amplitude-preserved seismic data.On the one hand,in view of the initial Vp/Vs model deviation,it’s difficult to invert accurate S-wave information.Thus,in this paper,we use the hybrid iterative inversion method,which can get the more accurate S-wave information by updating Vp/Vs model step by step.On the other hand,it’s difficult to build low frequency model because of the heterogeneous secondary carbonate.So it’s necessary to calculate fluid factor without low frequency information,and then we can reduce the risk causing by uncertain low frequency information through the combination of fluid factor and the former inverted P-and S-wave impedance,which can improve the inversion accuracy.Secondly,fluid has a small influence on the elastic parameters of saturated rocks because carbonate is characterized by high impedance value,which increases the difficulty of fluid prediction in time domain.Then the phenomenon is found that rocks have different dispersion of elastic parameter when rocks saturated different fluid,so we can also predict fluid by inversion on frequency domain.After inversion errors analysis and fluid sensitivity testing,the parameter I_αnamed P-wave dispersion gradient is selected to be new fluid factor,and it’s proved useful to discriminate fluid.Finally,in order to decrease the deviation causing by single inversion method,fluid is further classified by combining time and frequency domain inversion results,and the agreement with drilling can reach 82.2%.