婆罗洲北部岩石密度和磁化率特征及其对南海南部前新生界岩性识别的约束

Characteristics of density and magnetic susceptibility of lithology in northern Borneo and its significant of constraining on identification of Pre-Cenozoic rocks in the southern South China Sea

基于540块样品的体积磁化率-比重测量和矿物-岩性鉴定结果,分析了婆罗洲北部岩石类型、密度、磁化率特征,据此明确重磁资料对于南海南部中生界岩性识别的适用性.结果表明婆罗洲北部发育3类25种岩石,以发育中基性火成岩和外碎屑沉积岩为主,发育少量内源沉积岩、酸性火成岩和变质岩.具有极强磁性-强磁性,高密度-中等密度特征的岩石多为火成岩和凝灰质砂岩及其变质岩.弱磁性-无磁性、中等-极低密度特征的岩石多为砾岩、砂岩、粉砂岩、泥岩和煤等.弱-逆磁性、高-中等密度的岩石多为灰岩和硅质岩.新生界岩石具有密度较低、磁化率中等的特征.中生界岩石具有密度中等、磁化率中高的特征.前中生界岩石具有密度高、磁性低的特征.结合分布特征可知,前中生界岩石产生局部弱重力异常,中生界沉积岩产生区域性的负重力异常,火成岩产生区域性的正重力异常.南海南部的区域性高磁力异常来源于中生代的基性火成岩(及其变质岩),区域性中等磁力异常可能来源于酸性火成岩(及其变质岩).据此进行了南海南部中生界岩性的识别,结果表明南海南部存在大面积的中生界沉积岩分布区,认为南海南部中生界地层具有油气勘探潜力.

The rock type,density and magnetic susceptibility characteristics of northern Borneo were analyzed based on the results of bulk magnetic susceptibility values-specific gravity measurement and mineral-lithology identification of 540 samples.The applicability of gravity and magnetic data to identification of Mesozoic lithology in the southern South China Sea is clarified.The results show that the northern Borneo have sedimentary,igneous and metamorphic rocks that can be divided into 25 types.It is dominated by the intermediate-basic igneous rocks and exoclastic sedimentary rocks and followed by the endogenous sedimentary,acid igneous and metamorphic rocks.The characteristics of extremely strong magnetic-highly-magnetic,and high-medium density are presented by the igneous,tuff sandstone,and their metamorphic rocks.The characteristics of weakly magnetic-nonmagnetic,and medium density-very low density are presented by the conglomerate,sandstone,siltstone,mudstone,and coal.The characteristics of weakly magnetic-diamagnetic and high density-medium density are presented by the limestone and chert.Cenozoic rocks are characterized by low density and medium susceptibility.Mesozoic rocks are characterized by medium density and high susceptibility.Pre-Mesozoic rocks are characterized by high density and low susceptibility.Combined with the distribution characteristics of the rocks in the Borneo,Pre-Mesozoic rocks could generate weak negative gravity anomalies in limited area.The Mesozoic sedimentary rocks could produce regional negative gravity anomalies.The Mesozoic igneous rocks could produce regional positive gravity anomalies.The strong regional positive magnetic anomaly should be caused by the Mesozoic basic igneous rocks(including its metamorphic rocks)in the southern South China Sea.The medium regional magnetic anomalies should be generated by acidic igneous rocks(including its metamorphic rocks)in the southern South China Sea.The Mesozoic rocks of the southern South China Sea is interpreted by using the gravity and magnetic anomaly data according to the above results.The results show that there are large areas of Mesozoic sedimentary rocks in the southern South China Sea.It could be considered that the Mesozoic strata in this area have a good potential for oil and gas exploration.