Characteristics Of Sandstone Carrier And Mechanism Of Hydrocarbon Accumulation Of Lower Cretaceous Yingcheng Formation In Longfengshan-Dongling Area

Because of the abundant hydrocarbon resources in clastic rocks in China,the sandstone carrier as an important hydrocarbon migration channel and reservoir controls the process and spatial distribution of hydrocarbon.Indeed,it is important to study the characteristics of sandstone carrier and the mechanism of oil and gas migration and accumulation.The research takes sandstone carrier of Yingcheng formation in the south of Changling depression as the research object.Based on the study of diagenesis and hydrocarbon accumulation period,the coupling relationship between the physical evolution of sandstone carrier and hydrocarbon filling can be known.Combined with the results of the petrographic characteristic of inclusions and the evolution process of hydrocarbon phase state,we can rebulid migration process and establish a migration and accumulation model in the sandstone carrier.The sandstone carrier is mainly composed of gravity flow sediments in LongfengshanDongling subsag.Accroding to the fluid properties and their corresponding sedimental structure and tectonic characteristics,the gravity flow sediments can be divided into high-density turbidity,low-density turbidity,muddy density flow,sandy debris flow and muddy debris flow sediment.The sandy debris flow and muddy debris flow sediments were mainly developed in the inner subface of fan delta.The high-density turbidity,low-density turbidity and muddy density flow sediments were mainly developed in the outer subface of fan delta.The compaction ability of muddy density flow and muddy debris flow sediment is weak,and the pore-throat radius is relatively small.Although the porosity of sandy debris flow sediments is high,the fine grains are easy to deform and clog the throats,which leads to poor connection of pore-throats.The pore-throat radius of high-density turbidity and low-density turbidity sediment are relatively large.Different types of gravity flow sediments show great difference in physical property evolution.High-density turbidity and low-density turbidity sediments have strong anti-compaction ability,and the connected pore-throats provides the condition for acidic dissolution,so they can maintain good physical property during burial.The source rocks in Shahezi formation,which generally evolved to the mature stage,provide a stable hydrocarbon source for the oil and gas migration in the overlying Yingcheng formation.Based on the analysis of hydrocarbon generation history and aqueous inclusions,the result shows that the sandstone carrier of Yingcheng formation developed two stages of largescale oil and gas accumulation during burial.In the first stage of oil and gas filling(107?102Ma),the sandstone carrier still maintained good physical properties.Indeed,the sandstone carrier had encountered hydrocarbon charging and accumulation.The migration pathway was composed of the faults and sandstone carrier and hydrocarbon concentrated in the tectonic high part.In the second stage of oil and gas filling(96?65.5 Ma),the sandstone carrier had already been tight,and all faults had stopped acting.There are two types of inclusions in the Yingcheng Formation group in the study area,which are oil-gas inclusion and gas-water inclusion.The oil-gas inclusions were formed during the first stage of hydrocarbon charging,and the gas-water inclusions were formed during the second stage of natural gas charging.The two-phase components in gas-water inclusions contained methane,and the gas-water inclusions with gas content greater than 25% are difficult to be homogenized even when heated to the maximum value of the ancient geothermal temperature(160 ?),indicating the existence of dissolved and free gas.Along the growth direction of cementation,the content of gas phase in gas-water inclusions increased gradually under the conditions of room temperature(20 ?)and high temperature(160 ?).Indeed,it shows that the content of free gas increased gradually during hydrocarbon migration and accumulation.The study of the capture pressure of fluid inclusions shows that overpressure is generally developed in the sandstone carrier near the hydrocarbon generation center.Under the action of the remaining pressure,the natural gas overcoming resistance was mainly filled in the sandstone carrier near the hydrocarbon generation center.Because of the high pore-throat conectivity,the hydrocarbon mainly accumulated in the outer subface of the fan delta,and migrated along the outer frontier sand body to the tectonic high part.In the high part of the structure,because of the pore-throat connectivity,the natural gas was difficult to accumulate in the inner subface of fan delta.Due to the low migration and accumulation efficiency,the gas saturation of the inner subface of the fan delta is generally low.