Sandstones attributed to different lacustrine sediment gravity flows are present in the 7th and 6th members of the Yanchang Formation in the Ordos Basin, China. These differences in their origins led to different sand...Sandstones attributed to different lacustrine sediment gravity flows are present in the 7th and 6th members of the Yanchang Formation in the Ordos Basin, China. These differences in their origins led to different sandstone distributions which control the scale and connectivity of oil and gas reservoirs. Numerous cores and outcrops were analysed to understand the origins of these sandstones. The main origin of these sandstones was analysed by statistical methods, and well logging data were used to study their vertical and horizontal distributions. Results show that the sandstones in the study area accu- mulated via sandy debris flows, turbidity currents and slumping, and sandy debris flows predominate. The sand- stone associated with a single event is characteristically small in scale and exhibits poor lateral continuity. How- ever, as a result of multiple events that stacked gravity flow-related sandstones atop one another, sandstones are extensive overall, as illustrated in the cross section and isopach maps. Finally, a depositional model was developed in which sandy debris flows predominated and various other types of small-scale gravity flows occurred frequently, resulting in extensive deposition of sand bodies across a large area.展开更多
The carbonates in the Middle Ordovician Ma_5～5submember of the Majiagou Formation in the northern Ordos Basin are partially to completely dolomitized.Two types of replacive dolomite are distinguished：（1） type 1dol...The carbonates in the Middle Ordovician Ma_5～5submember of the Majiagou Formation in the northern Ordos Basin are partially to completely dolomitized.Two types of replacive dolomite are distinguished：（1） type 1dolomite,which is primarily characterized by microcrystalline（〈30 urn）,euhedral to subhedral dolomite crystals,and is generally laminated and associated with gypsumbearing microcrystalline dolomite,and（2） type 2 dolomite,which is composed primarily of finely crystalline（30-100 urn）,regular crystal plane,euhedral to subhedral dolomite.The type 2 dolomite crystals are truncated by stylolites,indicating that the type 2 dolomite most likely predated or developed simultaneously with the formation of the stylolites.Stratigraphic,petrographic,and geochemical data indicate that the type 1 dolomite formed from near-surface,low-temperature,and slightly evaporated seawater and that the dolomitizing fluids may have been driven by density differences and elevation-related hydraulic head.The absence of massive depositional evaporites in the dolomitized intervals indicates that dolomitization was driven by the reflux of slightly evaporated seawater.The δ～（18）O values（-7.5 to-6.1 ‰） of type1 dolomite are slightly lower than those of seawaterderived dolomite,suggesting that the dolomite may be related to the recrystallization of dolomite at higher temperatures during burial.The type 2 dolomite has lowerδ～（18）O values（-8.5 to-6.7 ‰） and Sr～（2＋） concentration and slightly higher Na～＋,Fe～（2＋）,and Mn～（2＋） concentrations and～（87）Sr/～（86）Sr ratios（0.709188-0.709485） than type 1 dolomite,suggesting that the type 2 dolomite precipitated from modified seawater and dolomitic fluids in pore water and that it developed at slightly higher temperatures as a result of shallow burial.展开更多
基金supported by the Science Foundation Programs(41302115)
文摘Sandstones attributed to different lacustrine sediment gravity flows are present in the 7th and 6th members of the Yanchang Formation in the Ordos Basin, China. These differences in their origins led to different sandstone distributions which control the scale and connectivity of oil and gas reservoirs. Numerous cores and outcrops were analysed to understand the origins of these sandstones. The main origin of these sandstones was analysed by statistical methods, and well logging data were used to study their vertical and horizontal distributions. Results show that the sandstones in the study area accu- mulated via sandy debris flows, turbidity currents and slumping, and sandy debris flows predominate. The sand- stone associated with a single event is characteristically small in scale and exhibits poor lateral continuity. How- ever, as a result of multiple events that stacked gravity flow-related sandstones atop one another, sandstones are extensive overall, as illustrated in the cross section and isopach maps. Finally, a depositional model was developed in which sandy debris flows predominated and various other types of small-scale gravity flows occurred frequently, resulting in extensive deposition of sand bodies across a large area.
基金supported by the Major National Science and Technology Projects of China (Grant No. 2011ZX05045)Sinopec (Grant No. 34550000-13-FW0403-0010)
文摘The carbonates in the Middle Ordovician Ma_5～5submember of the Majiagou Formation in the northern Ordos Basin are partially to completely dolomitized.Two types of replacive dolomite are distinguished：（1） type 1dolomite,which is primarily characterized by microcrystalline（〈30 urn）,euhedral to subhedral dolomite crystals,and is generally laminated and associated with gypsumbearing microcrystalline dolomite,and（2） type 2 dolomite,which is composed primarily of finely crystalline（30-100 urn）,regular crystal plane,euhedral to subhedral dolomite.The type 2 dolomite crystals are truncated by stylolites,indicating that the type 2 dolomite most likely predated or developed simultaneously with the formation of the stylolites.Stratigraphic,petrographic,and geochemical data indicate that the type 1 dolomite formed from near-surface,low-temperature,and slightly evaporated seawater and that the dolomitizing fluids may have been driven by density differences and elevation-related hydraulic head.The absence of massive depositional evaporites in the dolomitized intervals indicates that dolomitization was driven by the reflux of slightly evaporated seawater.The δ～（18）O values（-7.5 to-6.1 ‰） of type1 dolomite are slightly lower than those of seawaterderived dolomite,suggesting that the dolomite may be related to the recrystallization of dolomite at higher temperatures during burial.The type 2 dolomite has lowerδ～（18）O values（-8.5 to-6.7 ‰） and Sr～（2＋） concentration and slightly higher Na～＋,Fe～（2＋）,and Mn～（2＋） concentrations and～（87）Sr/～（86）Sr ratios（0.709188-0.709485） than type 1 dolomite,suggesting that the type 2 dolomite precipitated from modified seawater and dolomitic fluids in pore water and that it developed at slightly higher temperatures as a result of shallow burial.
