The Fillings And Hydrocarbon Of Ordovician Paleo-caves In Tahe Oilfield

Tahe oilfield is located in the north of Tarim Basin.As a karst fracture cavern reservoir,it attracts more and more attention for its huge reserves of over 100 million tons and rich karst phenomenon in the area.However,the characteristics and rules of filling fracture cavern are not clear,which seriously restricts the reservoir prediction and evaluation in this area.Aiming at the problem of cave filling existing in the study of karst ancient river,based on the description of underground river by geophysical technology,this paper focuses on the study of cave filling characteristics and filling geological laws of karst ancient river,gradually solves the bottleneck restricting the evaluation of karst ancient river potential,guides the effective prediction of cave storage space,and finally solves the problem of insufficient development potential in Tahe Area ?In this paper,through the description of fillings core,well logging response,seismic reflection characteristics,analysis and test,physical properties and productivity data analysis,the following knowledge is obtained: 1)four main fillings,namely,transport facies,collapse facies,seepage facies and chemical facies,are identified and corresponding well logging facies are established.(1)The transport lithofacies is the product of underground river,which can be seen as small to medium conglomerate,calcareous siltstone and argillaceous fine siltstone.On the logging,it shows that GR is medium to high right deviation,dentation or slight dentation,CAL is low right deviation,and porosity curve(den-ac-cnl)is dentated left deviation;(2)collapse lithofacies breccia has a single composition,and its logging response is gr left deviation,low dentation,and porosity curve is right deviation In the chemical filling phase,calcite or hydrothermal siliceous blocks are common in the core,and the logging response is not significant.(4)The percolation filling facies is mostly filled in the developed fracture and the gap between the breccia.The lithology is mainly gray green percolation argillaceous sand,light gray calcareous mudstone and calcareous siltstone,without bedding.The logging response is characterized by fingering or toothing to the right under the background of GR to the left,and the porosity curve is low amplitude to the left.2)The filling structure of the cave was dissected.Most of the underground river caves are of "multi-layer" filling structure,commonly including bottom collapse /breccia transportation + middle transportation sandstone + upper transportation sandstone or unfilled filling structure;most of the non underground river caves are of "double-layer" or "single-layer" filling structure,mainly including transportation sandstone and seepage filling,and unfilled caves are also developed.3)The filling age identification template is established.The gray green,light gray green,gray green sand,mud,calcium carbonate sand and their mixed filling materials are filled in the burial period(Early Hercynian),while the gray variegated breccia,with complex brecciacomposition,is generally the product of the construction period(Caledonian).4)The geological law of cave filling is revealed.(1)On the plane,with the decrease of paleohydrodynamic gradient,the development frequency of transport facies in the lower part of the river becomes higher and the thickness increases,and the unfilled caves are mainly developed in the upper and middle sections;(2)on the section,transport facies and unfilled facies are mostly developed in the seepage zone and runoff zone;(3)the underground river caves are of large scale,and transport facies are mostly developed under the action of runoff,and they are more prone to collapse,so collapse facies The transport facies are mostly filled in the underground river caves,while the non underground river caves have small scale and high development degree of seepage facies.5)The significance of oil and gas response of the cave is put forward.The cave filling itself has certain reservoir property,but it is not the main reservoir space of the Ordovician cave;the karst collapse body exists and has a large contribution to the productivity,but the collapse is not the only factor of the productivity scale.