The Influence Of Magmatic Activity On The Alteration Of The Sandstone In The Petroleum-bearing Basin

There are forty-six Mesozoic and Cenozoic petroleum-bearing basins inNortheast China. They belong to various types and mainly formed and developedin the Mesozoic. As an important section of circum-Pacific volcanic belt, theNortheast China saw frequent and intensive magmatic intrusion and eruption inMesozoic period. The Mesozoic volcanic rock is extensively distributed in bothmountains and Mesozoic fault basins, which is controlled by fault belts extendingin NW and NE direction in the basement, especially the joint of fault belts. Suchimportant petroleum-bearing basins as Songliao, Sanjiang and Jixi are affected bymagmatic activities at different extent.At present, studying on diagenesis of heat convection caused by the magmaticactivity in petroleum-bearing basins in East China is a key to the research of boththe evolution fluid and diagenesis in basins. To explain the effect of magmaticactivity on oil reservoir rocks and its contribution to diagenesis, three questionshave to be considered. The first is the type and occurrence of authigenic minerals.The second is the alteration of epithermal magmatic activity on diagenesis of thesemi-solidified or solidified sandstone. The third question is the effect of diagenesisof heat convection, caused by the magmatic activity, on reservoir characteristics ofthe sandstone. The Songliao, Sanjiang and Jixi basins are favorable for the solutionof these questions.In the southern Songliao basin, dolerite veins intrude the semi-solidifiedsandstone of the Yingcheng formation. The abrupt intrusion of dolerite veinscaused the pore-almond structures in the semi-solidified sandstone wall rock.Moreover, the magmatic intrusion results in brittle fractures in clastic grains around,then these fractures can be closed along with continuous increasing of temperature.However, due to continuous pressure and high temperature, the pressure at jointpoint of different quartz grains becomes more and more concentrated, which causesthe compaction dissolution and comb quartz grains surrounding the primary quartz.The dissolution of the quartz under the high temperature causes increasing ofsilicon content in the fluid in holes. Owing to fluid diffusing, the fluid rich insilicon flows from the high-energy zone to the low-energy one and results in thedendritic minicrystal quartz by recrystallization. More and more farther fromcontact belt of intrusive bodies, minicrystal quartzes get less and less, shorter andshorter as well as larger and larger. In addition, the content of enlargement in thequartz becomes more and more.In Zhangxin coal deposit in Jixi basin, the gabbro porphyrite intrudes thestrata of Chengzihe formation. In the gabbro porphyrite, the growing zone of thelaumontite can be found. The laumontite is contemporary with the authigenicsericite and lode carbonate minerals in the sandstone. All those are resulted fromthe same magmatic hydrothermal activity. The thermometamorphism taking placein the contact zone between intrusive rock and sandstone caused suchlow-temperature metamorphic minerals as authigenic sericite, biotite. Thanks to thepressure caused by abruptly intruding of a mass of magma, thin fractures formed inthe sandstone of Cengzihe formation covering the gabbro porphyrite body inZhangxin coal deposit in Jixi basin. The distribution of carbonate cement in thesandstone, isotopic data of carbon and oxygen and the homogeneous temperature ofenlargement of quartz can confirm the hydrothermal activity.In Suibin depression region in Sanjiang basin, frequent volcanism can beproved by extensive occurrence of volcanic bodies and subvolcanic ones. There areandesite and andesitic porphyrite in strata of Dongrong formation, alteratedandesite in strata of Chengzihe formation. High-value abnormities of thereflectance of vitrodetrinite show that intensive volcanic activities affected thepaleogeothermic field. The volcanism changed the diagenetic system by heatingformation water and resulted in such authigenic minerals as muscovite, biotite,quartz and illite. The autigenic muscovite is mainly located in pores and fracturesin clastic particles. The amount of muscovite is high up and below the volcanicrock, and it becomes lower and lower far from volcanic rocks. The occurrence ofvolcanic rocks and authigenic minerals, abnormities of the reflectance ofvitrodetrinite indicate the hydrothermal activity in Suibin depression region. Thestudying on the distribution of authigenic minerals and thin fractures shows that theamount of muscovite and calcite is higher near the volcanic rocks. It may be provedthat the diagenesis caused by heat convection can bring the reallocation ofinterstitial materials in reservoir rocks as well as anisotropism of these rocks.The K-Ar isotopic ages of authigenic illite from Jixi and Sanjiang basins areless than those of strata around, which suggests that hydrothermal activities takeplace in late stage of the early Cretaceous period and be associated with the heatingcaused by the magamtism in Dongshan and Songmuhe period.