Dolomite Origin of Lower Ordovician in Hetian River Gas Field, Tarim Basin

Key words: 
• Tarim Basin /
• Lower Ordovician dolimite origin /
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Abstract: The Hetian river gas field is a new and large gas field in Mazatage fault belt of Tarim basin. Ordovician carbonate rock is principal production formation. Lower Ordovician is characterized by developed dolomitization which can greatly improve reservoir property. There are two types of dolomite in Lower Ordovician. The bedded dolomite is in lower part. Their features is fine crystal, cathodolaminescence of violet red and blue violet, degree of order 0.57～0.68, low Sr and Na content, and negative carbon oxygen isotope value, implying that the bedded dolomite was formed in environment of early shallow buried fresh water and seawater mixing. On the contrary the upper part patch calcareous dolomite is medium crystal, degree of order 0.85～0.90, high Sr content, low Na content, and more negative value carbon oxygen isotope indicating that the patch dolomites was formed in environment of late deep buried compaction flow and fresh water mixing. The bedded dolomite was formed in an environment of restricted platform,where water was confined and salinity was high.There was porous water of high salinity with high Mg/Ca in intergranular pore of orginal sediment. It has much higher carbon oxygen isotope value and trace element contents than normal seawater. During burial, there was a lot of fresh water down from east northern Tabei old land along slope to this area. Fresh water and porous seawater with high salinity will be mixed in sediment, resulting in the dilution of pore water salinity, value of carbon oxygen isotope and trace element will decrease. But the Mg/Ca ratio will keep unchanged. So dolomitization will take place. Late Ordovician in Mazatage fault belt is open platform facies. Salinity of seawater is normal. Over lower Ordovician sediment is mainly mud limestone and mud intercalation limestone. When mud limestone was buried over 2 000 m, montmorillonite in clay mineral transferred to illite. A lot of ion of Sr, Fe, Ca, Mg and so on came into pore fluid. Because of gravity, pore fluid will flow downwards from Tabei old land along the Sajingzi fault. Two types of flow mixed in grain limestone of upper lower Ordovician. CaMg(CO 3) 2 was over saturation. Patch calcareous dolomite will be formed.Two types of dolomitization can be formed in mixed water, but the Mg ion sources are different. The later is aspecial genetic model.