Micro-characteristics of Paleogene Lacustrine Petroleum Source Rocks in Jiyang Depression

Key words: 
• micro-characteristics /
• petroleum source rocks /
• Paleogene /
• Jiyang depression

Abstract: The Jivang depression is one of the suhhasins of the Bohai Gulf Rift Basin in the eastern China.About 65 oil fields have been discovered in this depression over the past 30 wars and the oil mainly derived from three types of finch laminated lacustrinc rocks of Shahcjic Formation （Eoccnc-Oligoccnc）.Thcsc source rocks are organic-rich laminated shale calcarcous laminated shale and calcareous laminated mudstonc.High resolution studies have been carried out on these petroleum source rocks by scanning electron microscopcC（SEM),hackscattcrcd electron imagcry（ BSEI)and energy dispcrsivc spectrometry(EDS).The results show that these source rocks are all laminated sedimentary rocks, and organic matters are very abundant in it, but they have the different sediment composition. Organic-rich laminated shale is composed of organic matter lamination and clay mineral lamination: The clay-rich laminac range in thickness from 15 to 100μm and are predominantly composed of illite, (80 %).In addition they have subsidiary components which comprise ankcritc( 5-10%),pyrite ( 1-3%),quartz (l-2%)and fcldspar( 1%-2%)along with trace amounts of apatite( 0.03%).In contrast, the organic-rich laminae are thinner than the clay-rich laminae, ranging from 6-50 μm and are predominantly composed of amorphous organic mattcr( 20%-70 %),as well as organic matter these laminae also contain dctrital clay(predominantly illite),silt-sized quartz, fcldspar( 1%-2%),and pyritc( 1%-3%).Displacivc calcite layers were also present.The calcite in these layers consits of low-Mg, non-fcrroan calcite.In addition to the characteristic "beef" texture of this calcite these layers also contain pyrite framhoids, dctrital quartz and ankcritc.Calcareous laminated shale is composed of organic matter lamination and coccolith or coarse calcite lamination:The coccolith laminae laminae is composed almost wholly of one species, Rcticulofcncstra hohaicnsis and the coarse grained calcite laminc are composed of coarse calcitc( 10-20 μm) and also contain coccolths.within the coccolith laminae the size of coccolith ranges from 2 to 5 μm,and it has a good preservation.within the coarse calcite laminae the coarse calcite are sparrv calcite characteristically by its overgrowth and the coccolths have a poor preservation characteristics such as dissolution.This indicated that the coccolith-hearing coarse calcite laminae may he rvstallizcd from the dissolution of coccolth laminac.Organic matter laminae is mainly composed of the lath of amorphous organic matter and contain trace amounts of dctrital clay.The thickness of organic-rich laminae range from 15 to 50 μm and the thickness of the calcite laminae range from 40 to 130 μm. Another important phenomenon is that there are dinoflagcllatcs laminae in the organic-rich laminae.These dlnoflagcllatcs laminae are also composed of one species Fcramo( Fig.3). Calcareous laminated mudstonc is composed of finely grain calcite lamination and organic-rich clay mineral lamination.The clay-rich laminae range in thickness from 50-250 μm and are predominantly composed of illite(70%)and organic matter(organic matter may he formed by suhlaminac in clay-rich laminae ).They also contain fine grained calcitc( 5 %) and silt-sized dctrital quartz( 3 %).In contrast the carbonate-rich laminae are predominantly composed of micrrn sized and fine grained calcite which form layers 30-100 μm in thickness.The dinoflagcllatcs laminae is also discovered in the organic-hearing clay-rich laminae( Fig.6).