The Geochemical Characteristics Of Ordovician Paleokarst Reservoirs In Tahe Oilfield

Tahe oilfield is the first big Palaeozoic marine oilfield of China, and it has obtain a breakthrough in Paleozoic Marine carbonate oil and gas exploration. Ordovician carbonates is the main reservoir, so it will make huge economic value to research Ordovician reservoirs in tahe area by geochemical method.Besides learning previous research achievements, author analyzed the rock sample from twenty drilling wells. Concrete analysis methods include carbon isotope analysis, oxygen isotope analysis, strontium isotope analysis, Cathodeluiminescence, electronic probe, trace element analysis and fluid inclusion analysis. Author has get some opinions as follows:1 Rock types of Ordovician carbonates in Tahe area is correlated with era. Limestone Mainly distributed in Lianglitage formation, Qiaerbake formation, Yijianfang formation and Yingshan formation. Dolostone Mainly distributed in Penglaiba formation.2 On the whole, oxygen isotope and carbonate isotope of carbonate minerals in pore spaces or fissures are lower than Ordovician limestone. strontium isotope, iron content and manganese content of carbonate minerals in pore spaces or fissures are higher than Ordovician limestone. It shows that carbonate minerals in pore spaces or fissures were subjected to fresh water.3 Inclusion data shows that at least six types of fluids are involved with Ordovician carbonates in Tahe area.4 The change of iron content and manganese content in calcite vein and calcite in pore is related to the distance with unconformities. Author use the change to define depth of paleokarst influence. The depth of paleokarst influence is 130 meters.5 There are three unconformities which contribute to carbonate reservoirs: the boundary surface of middle-upper Ordovician Qiaerbake formation and Yijianfang formation in Caledonian's first act, the boundary surface of upper Ordovician Sangtamu formation and Lianglitage formation in Caledonian's second act, the boundary surface of carbonic rock formation in early Hercynian and Ordovician rock formation. karst product formed in middle Caledonian locates at south of Tahe oilfield, and it isn't affected by karstification in early Hercynian because of Sangtamu formation. karst product formed in early Hercynian locates at north of Tahe oilfield.6 Manganese content and strontium isotope of the karst product formed in Caledonian's first act is lowest. Manganese content and strontium isotope of the karst product formed in Caledonian's second act is lower than the karst product formed in early Hercynian. Carbonate isotope of the karst product formed in Caledonian's first-second act is higher than the karst product formed in early Hercynian. Form above comparison, we can draw the proper conclusions. The karst product formed in Caledonian's first act is not correlated with organic, and the time of exposure of it is very short. If the Sangtamu formation is very thin and huge faults cut across the formation, Lianglitage formation will has higher strontium isotope and lower carbonate isotope. In Hercynian, Because Ordovician carbonates and Devonian-Hercynian siallite rocks are both above the ground, the karst product formed in Hercynian has higer strontium isotope. Due to long time's soluting in Hercynian, the karst product has lower carbonate isotope.7 Ordovician carbonates in Tahe area also suffered buried solution. Inclusion data shows that some carbonate minerals in pore spaces or fissures near unconformities are connected with high temperature fluid even hydrotherm, and that there is high temperature dolomite in Ordovician Penglaiba formation, which might associated with huge fault in west Tahe area.8 Due to far away from unconformities , carbonates in Penglaiba formation and bottom Yingshan formation didn't suffer epiclastic karst in mid-Caledonian and early Hercynian. But carbonates in Penglaiba formation and bottom Yingshan formation suffer retrograde solubility because of tectonic uplift. The other way to make arbonates in Penglaiba formation and bottom Yingshan formation solubility is fluid moving along faults formed in tectonic uplift. Because Tahe basin subsides all the time after Hercynian, the temperature of rock strata steady increases. Reservoir space in Tahe oilfield has blocked up because of plusΔT.