Sand Body Characteristics And Sedimentary Process Reconstruction Of The Jurassic Shaximiao Formation In Longgang Area, Central Sichuan

The Jurassic Shaximiao formation sandbody of Central Sichuan Basin Longgang Area is widely developed but has unclear in sandbody characteristics and distribution.In order to identify the channel sandbody evolution process,the research are conducted by the integrated of field outcrop,core observation,thin section and logging-seismic prediction.The results show that Shaximiao in longgang area is primarily river-lake sedimentary environment,sedimentary facies belt such as braided river,alluvial flat,beach bar and mudflat could be identified.The Sha-1 channel sand bodies are mostly belongs to braided river deposition.The lithology majorly comprises of gray and purple-red coarse sandstone,with little amount of medium sandstone and conglomeratic sandstone of mostly thick-layer massive sand-bodies.On the whole,the maturity is relatively low.The sandbody rock types of Longgang Area Shaximiao formation are mainly feldspar lithic sandstone and lithic feldspar sandstone.The major component of sandstone clastic particles is quartz,followed by feldspar and rock debris.The sandstone can be divided into coarse and medium sandstone and is separated to moderate-good with sub-rounded rounding,and point-line contact and particles support among particles majorly.Meanwhile,the major cementation type is porous cementation,while partial layer section has developed chlorite clay mat and formed diaphragm type servomotor cementation and partial basal and regenerative cementation.The accumulation spaces of channel sandbody include original pore,secondary pore which are mainly interparticle dissolution pore,interparticle dissolved pore and construction joint.Since the lower-middle part has relatively higher feldspar contents,the interparticle dissolved pore is developed correspondingly.The major porosity of channel sandstone is within 1.5?8%(average value 2.8%)while the permeability of the sandstone is within 0.1?1mD(average permeability 0.3 mD).And it possesses the reservoir characteristics of partial developed relatively high porosity(?>6%)under low porosity and low permeability background.The pore throat structure has the characteristics of high displacement pressure,high-moderate pressure,high mercury unsaturation volume,small maximum intercommunicating pore throat radius and mid-value pore throat radius,and low mercury withdrawal efficiency.The reservoir is obviously controlled by sedimentary and secondary corrosion effects.While the existing of fracture is an important condition to gain industrial oil and gas flow.In the background of lake basin shrinkage in Lianggaoshan peroid and extensive lake dryout,since the western part of the basin uplifts relatively,the western and southwestern sedimentary basements uptrends largely and correspondingly while the lake water secedes in the south west-north east direction.The Longgang Area locates at an area adjoining to depocenter and has formed multi-sedimentary of floodplain-river and gradually developed channel sandstone based on down residue multi-sedimentary of shore-shallow lake.The Sha-1 phase channel sandbody has high width box pile-ring type logging response characteristic,while the earthquake section shows as event pulldown and lentoid and is in stripped distribution in graphics.The sandbody width is about 390?3440m(average:1170.8m),thickness is 4?33.75m(average:13.74m)and the width-thickness ratio is 26:1?312:1(average:106:1).There have developed five-period channels in the 200m range up from Sha-1 phase and the channel sandbody graphics change frequently.The channel sandbody geometry quantitative analysis show that Longgang Sha-? channel belongs to low camber(average camber:1.044)braided river and the channel width is about 0.6?2.8km.It has frequent graphics swing because of the influence of north-east provenance.Unlike the third-phase channel which has insufficient source supply at sometimes,other channels sandbodies generally manifest the gradual development rules.The channel sandbody distribution range and development degree within the 100-200m range up to Longgang Area Shaximiao formation,while the sandbody develops best in the Longgang 9 well area of Longgang Area which possesses good connectivity.Its better matching relation with fault or fracture can facilitate the oil and gas reservoir formation.