A lot of large and medium lithologic-type fields have successively been found in the slope areas of Ordos

Songliao

Bohai Bay and Junggar basins in last ten-old years.Their formation need some essential conditions

including:1) development of high-quality source rocks

with moderate to high abundance of oil

&

gas resources(0.1～0.6 million t/km

2

); 2) development of relatively large hydrocarbon-gathering units to ensure large amount of oil and gas could fully migrate toward the same structural

stratigraphic and lithlogic belts;moreover

the structures developed on the gentle slopes of large basins would relatively be uneasy to be cut off

and sandbodies would be able to distribute evenly in large area;3)sandbodies deposited in the fandelta front would not only provide huge reservoir spaces;since they are unevenly-roughly envenly interbedded with high-quality souce rocks

they and source rocks together would also constitute high-efficient fluid-discharge units increasing the abundance of oil and gas resources in various blocks;4) because lithologic wedging-out belts are characterized by repeated swinging and migrating

lithologic traps would not be able to stack vertically one upon another

and would be difficult to become high-abundance fields;however

in the area characterized by abrupt change in occurrences

such as in regional fault zone

uplifted area at depth or stratigraphic overlapping wedging-out belt

there would be good stacking lithologic reservoirs; 5)reversed tilting would often reform the frontal zone of sandbodies from downdip pinchout to upwarp pinchout

while uplifted tilting would result in the lateral blocking of sandbodies

which would be favorable to the development of large and medium lithologic-structural composite traps;6)the strikes of fan delta sandbodies would often be perpendicular or obliquely cross the course of strata at large angles;as a result

it would not only ensure the opening of delivery system

but would also be favorable to the formation of numerous lithologic traps on the upwarping sandbodies;7)the reservoir with low and moderate porosity and permeability would be the most favorable to the formation of lithologic oil and gas reservoirs

because the reservoirs in the fan delta front have generally experienced relatively strong diagenesis and catagenesis

i.e.the thin siltstones and fine sandstones in the interdistributary bay would have seriously been tightened

which would block the further migration of oil and gas;8)the timely matching of trap formation and hydrocarbon migration would be favorable to the formation of large and medium lithologic oil and gas fields.