Accumulation And Preservation Conditions Research On Marine Shale Gas Of The Lower Cambrian In Huangping Sag And Its Periphery

This study focused on the shale gas accumulation characteristics of Jiumenchong formation(Lower Cambrian)around Huangping sag,such as shale distribution,organic geochemistry,reservoir and gas-bearing.This paper also has analyzed the influence of the development characteristics of cap rock,faulting,uplift and denudation,magmatism and metamorphism on the preservation conditions.Besides,the ancient hydrocarbon preservation conditions and the present hydrocarbon preserving conditions are discussed from the point of view of the fluid.Based on these works,the comprehensive evaluation indexes of shale gas accumulation and preservation in the study area are proposed,and the favorable area of shale gas is optimized.The results show that:(1)The thickness of shale in the study area is ranging from 21m to 438m,and most areas are greater than 100m.The burial depth of the shale is basically less than 3500m.The type of organic matter is ?1.The TOC is ranging from 0.29%to 10.12%,with an average value of 5.51%.The R0 is ranging from 1.86%to 3.29%,with an average value of 2.57%.(2)The shale minerale compositions are dominated by quartz(average value is 49.7%),feldspar(average value is 16%)and clay minerals(average value is 16.4%).And the main composition of clay minerals is illite.In the distillation extraction environment,the porosity measured by the GRI is ranging from 3.54%to 5.52%,with an average value of 4.88%.There are many types of reservoir space in shale which can be divided into matrix-related pores and cracks,the former can be further classified into intergranular pores,intrapartical pores and organic matter pores and the later can be subdivided:into explicit cracks and micro cracks.(3)The multiple drilling in the study area has good gas shows in Jiumenchong formation.The methane adsorption capacity of shale is ranging from 1.45m3/t to 5.27m3/t,with an average value of 3.47m3/t.The average amount of desorbed gas of Huangye 1 well in Jiumenchong shale is 0.89m3/t.(4)The shale has good cap rock,roof plate and bottom plate conditions,but the preservation conditions suffered some damage because of the complex tectonic movement in the study area.Uplift and denudation are the main factors for the destruction of preservation conditions.Yanshan and Himalayan period is the most prominent movement,which make the denudation thickness in the study area is more than 3000m.And there is some erosion in Duyun and Guangxi movement.Fracture also has some damage to the preservation conditions.However,the damage of magmatism and metamorphism is not obvious to the preservation conditions.(5)The analysis of carbon and oxygen isotopes and fluid inclusions indicate that a major period of paleo-fluid activity in the study area is the Mid-Late Yanshan to Himalayan,especially in Creaceous and Late Cenozoic.Multiple strata of the study area suffered from atmospheric water infiltration that mainly caused by Himalayan movement,with older ones in better conditions compared to new ones.The paleo-fluid activity during Yanshan to Himalayan period is further evidenced by the tectonic background and the characteristics of the plane distribution of the denudation in this period.The depth of the paleo-atmosphere water infiltration in the study area is about 1400 to 4100 meters,with a relatively small depth of infiltration in Yuqing,Huangping-Kaili-Majiang and so on.(6)The analysis of stratigraphic water indicates that when the depth in the study area is less than 800m,the whole area is in free exchanging zone.And when the depth is more than 2200m,the shale gas in Jiumenchong formation already has good preservation conditions although it is in exchanging-inactive zone.The circulation depth of thermal water ranges from 300m to 1800m,of which the circulation depth is less than 400m in north of Zhenyuan and west of Fuquan.And the chemical properties of thermal water indicate that the preservation conditions in the location of the thermal water circulation have basically destroyed.(7)The comprehensive evaluation indexes system of shale gas accumulation and preservation conditions in the study area is explored.Based on this,Huangping-Fuquan-Kaili,Shibing-Zhenyuan and so on are pointed out as shale gas favorable areas.