The nation's great oil conglomerates are secretly preparing for a government assault on their economic power. They are anxiously measuring the effect a possible breakup of their corporate structure might have; some companies are actually formulating contingency plans for complying with an antitrust ukase.

"I expect if we didn't, the shareholders would throw us all out," one high-level corporate source acknowledged. Another admitted his firm has conducted seminars on what would occur from an antitrust crackdown.

In public propaganda, the oil monsters moan that divestiture (the fancy name for a breakup) would destroy them and throw the nation's delicate energy system out of kilter. But computer projections being studied in their board rooms tell a different tale, with possibly even greater profits for the dominant producers.

Two types of divestiture are being evaluated. Horizontal divestiture would break Big Oil's present stranglehold on the development of other energy sources such as coal, shale oil, nuclear generators, solar devices, geothermal power from underground heat and even energy generated by tides or garbage. Vertical divestiture would mean whittling down the start-to-finish control of the petroleum corporations over oil and gas. They could be compelled to divorce oil fields from the refineries or perhaps the tanker shipments and pipeline deliveries from the retail-sales outlets.

Impact studies, some more detailed than others, of the consequences of executives are already in the hands of executives of Amoco, Arco, Exxon, Gulf, Mobil, Shell, Standard of California. Sunoco and Texaco among others. The industry is carefully keeping many of them secret.

For the computer printouts show that some, perhaps most, of the oil consortiums could actually turn a profit by cutting loose their company owned outlets. Of course, this might deprive them of greater profits in years to come should the dwindling oil supply send prices skyrocketing . The oil companies, therefore, want to keep their gas stations.

But the computers make it clear that not a single oil company would go bankrupt by selling gas stations to independent local owners. Similarly, the studies show that the oil producers could sell their pipelines and tankers at a reasonable profit.

The printouts also indicate that Big Oil could spin off its interests in shale, solar energy and geothermal development without disastrous consequences. The loss of coal, nuclear plants and uranium mining would not be fatal surgery. Indeed, the computers predict handsome earnings for the sale of subsidiary operations, except in two instances.

The computers warn they probably could not survive in their present corporate form if an antitrust barrier were placed between ownership of both petroleum and natural-gas fields. The same would hold true if the production of crude was severed from the refinery process.

The handwriting on the wall is already, visible, meanwhile, with some steps being taken in possible anticipation of government action. For instance, Gulf and Sunoco have moved their production and marketing divisions apart. Exxon has moved in the same direction with its solar energy program. Shell and Texaco have done likewise with their pipelines, Contimental and Mobil with their coal holdings. While these moves were taken in the name of business efficiency, they have created some natural fracture lines along which the petroleum industry could be split.

President Carter made an emphatic campaign vow to consider forcing the oil crowd to give up their retail outlets. But that was before he got to the White House. With Energy Secretary James Schlesinger pulling strings, the promise has been mislaid, and the Justice Department is moving at a snail's pace toward any antitrust action against the petroleum companies.

In Congress, the late Sen. Phil Hart (D-Mich.) scared the tar out of the industry by nearly winning Senate approval of a divestiture bill. Currently, Rep. Morris Udall (D-Ariz), a defeated Carter primary rival, is painstankingly trying to move a limited divestiture through the house.